Picking a transistor for low output impedance circuit...

[amdx]

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

 

[whit3rd]

On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, amdx wrote:

Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.

Well, it\'s a Darlington; the impedance goes down when you
either use a higher bias current, or replace a transistor with a
germanium one...

Since this is AC-coupled, can you use a stepdown transformer
to do the coupling? That will get the impedance lower... so
would some positive feedback.

 

[amdx]

On 10/5/2022 8:24 PM, whit3rd wrote:

On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, amdx wrote:
Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
Well, it\'s a Darlington; the impedance goes down when you
either use a higher bias current, or replace a transistor with a
germanium one...

Since this is AC-coupled, can you use a stepdown transformer
to do the coupling? That will get the impedance lower... so
would some positive feedback.

 Do you mean on the input?
Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω?
                            Mikek

 

[Ricky]

On Wednesday, October 5, 2022 at 9:43:03 PM UTC-4, amdx wrote:

On 10/5/2022 8:24 PM, whit3rd wrote:
On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, amdx wrote:
Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
Well, it\'s a Darlington; the impedance goes down when you
either use a higher bias current, or replace a transistor with a
germanium one...

Since this is AC-coupled, can you use a stepdown transformer
to do the coupling? That will get the impedance lower... so
would some positive feedback.
Do you mean on the input?
Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω?

The output impedance of your circuit is controlled by the current gain of the two transistors and the resistors R4 and R5. Use transistors with higher current gain and/or adjust the resistor values to put the transistors on higher current gain portions of their curves. More current gain provides more current capability to reduce the output impedance.

As was already asked, the transformer actually sets the output impedance from the amplifier stage. Why not change that to give lower output impedance? Lower impedance, but lower voltage. Do you know the turns ratio? Is it 1:1?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[John Larkin]

On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz,
on up to 7.70? at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

A fast opamp would be better.

Or a buffer, like BUF602. It\'s typically 1.4 ohms at 10 MHz.

 

[whit3rd]

On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, amdx wrote:

Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.

Would like to see it lower, especially at the high frequencies.

Another thought: PNP output transistor (NPN/PNP pseudodarlington circuit variant) might help; the
base spreading resistance in the output stage is higher for P-doped base than
it is for N-doped.

2N3866 is in TO-39 case? That\'s OLD silicon. How about 2SA2124 or CZT955?
The emitter output impedance is the low-Z target, I\'d guess; for positive feedback, you
can run emitter current through a bead with a secondary winding to boost the RF base
current... but gently. I\'d want to SPICE it before committing that to a power-up test.

 

[amdx]

On 10/5/2022 9:00 PM, Ricky wrote:

On Wednesday, October 5, 2022 at 9:43:03 PM UTC-4, amdx wrote:
On 10/5/2022 8:24 PM, whit3rd wrote:
On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, amdx wrote:
Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
Well, it\'s a Darlington; the impedance goes down when you
either use a higher bias current, or replace a transistor with a
germanium one...

Since this is AC-coupled, can you use a stepdown transformer
to do the coupling? That will get the impedance lower... so
would some positive feedback.
Do you mean on the input?
Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω?
The output impedance of your circuit is controlled by the current gain of the two transistors and the resistors R4 and R5. Use transistors with higher current gain and/or adjust the resistor values to put the transistors on higher current gain portions of their curves. More current gain provides more current capability to reduce the output impedance.

As was already asked, the transformer actually sets the output impedance from the amplifier stage. Why not change that to give lower output impedance? Lower impedance, but lower voltage. Do you know the turns ratio? Is it 1:1?

 I was hoping someone would have a part number for a transistor higher
current gain than a 2n4401 and/or a different output transistor.
The transformer is 50 to 1, 2500 impedance ratio. The one turn secondary
sees roughly 0.3Ω to 3Ω.
The circuit draws 117 mA; the output transistor draws 84 mA.
Driving with a 50Ω signal generator, the change of the R on the
secondary, reflected back to the primary caused a 2% reduction in the
drive voltage, so
it was thought the HP impedance converter will correct a lot of that error.
                                  Mikek

 

[bitrex]

On 10/5/2022 10:15 PM, John Larkin wrote:

On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz,
on up to 7.70? at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

A fast opamp would be better.

Or a buffer, like BUF602. It\'s typically 1.4 ohms at 10 MHz.

If OP is committed to discretes there\'s always the classic White follower:

<https://norcim-rc.club/Radio15_files/image025.jpg>

Which should have substantially better output impedance at high frequency.

(You can make one of the output transistors PNP and add a third
transistor of either polarity and eliminate the coupling capacitor)

 

[Phil Hobbs]

amdx wrote:

Hi all, I\'m following a group discussing building a Q meter. HP has a
two transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0


One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at
1MHz, on up to 7.70Ω at 30MHz after trying several common transistors
for the first transistor. The output transistor is a 2n3866. The
first transistor was a 2n4401,this gave the lowest output impedance.
I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit. Would like to see it lower,
especially at he high frequencies. Mikek

In the usual technician-course model, The output impedance of a BJT
follower at lowish frequency is basically the sum of two terms: first,
the small-signal emitter resistance (25 ohms/I_C in mA), and second, the
impedance seen by the base divided by the AC beta.

Both stages run pretty hot, with Q1\'s I_C ~ 20 mA and Q2\'s around 85 mA
give or take. Q1 therefore has a Zout of a bit less than 2 ohms, based
on an AC beta of 40 or 50 and the 25-ohm impedance at the base of Q1.
(I assume that J6 normally connects to a 50-ohm source, but it doesn\'t
make much difference.)

So the source impedance seen by Q2 is very small, so that contribution
to its Zout is small squared. The output impedance in the simple model
is thus 25 ohms / 85 mA, about 300 milliohms.

In real life, besides AC effects, there are extrinsic resistances in the
base and emitter of Q2 to consider, which are usually on the order of an
ohm in medium-power devices.

Q2 has a quiescent dissipation of around (25V*562/2612 + 0.7)**2 / 220 =
170 mW, so you\'re probably looking at a TO-5 can device. BITD my go-to
for that sort of job was the 2N5109.

A 2N4401 is reasonable, but its f_T is only about 300 MHz. From things
in my junkbox, I\'d probably start with a BFU520A or BFS17 and rebias it
a bit to reduce the dissipation, maybe with a parallel RC in its collector.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Gerhard Hoffmann]

Am 06.10.22 um 01:51 schrieb amdx:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0

One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.

The 2N3866 was an overlay transistor as RCA used to call it.
Every emitter stripe had a NiCr resistor in series to balance
the current. That may work against you.

Cheers, Gerhard

 

[amdx]

On 10/6/2022 1:22 AM, Gerhard Hoffmann wrote:

Am 06.10.22 um 01:51 schrieb amdx:
Hi all,
   I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0

One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at
1MHz, on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
    I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
  Would like to see it lower, especially at he high frequencies.

The 2N3866 was an overlay transistor as RCA used to call it.
Every emitter stripe had a NiCr resistor in series to balance
the current. That may work against you.

Cheers, Gerhard

Thanks for that info, however, I found that the builder actually used
the 2N5109.
I don\'t have any 2N5109s, but I do have 2N3866s, so I may need to
rethink it\'s use.
                                        Mikek

 

[Fred Bloggs]

On Wednesday, October 5, 2022 at 7:51:12 PM UTC-4, amdx wrote:

Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
Would like to see it lower, especially at he high frequencies.
Mikek

Is the coupling stepdown a powdered iron? The core loss from 1-30 MHz is possibly very significant and probably responsible for the coupling loss. The frequency dependence is a giveaway. It\'s only a 1.25 dB deviation- which is not significant for RF.

 

[amdx]

On 10/6/2022 10:47 AM, Fred Bloggs wrote:

On Wednesday, October 5, 2022 at 7:51:12 PM UTC-4, amdx wrote:
Hi all,
I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
Would like to see it lower, especially at he high frequencies.
Mikek
Is the coupling stepdown a powdered iron? The core loss from 1-30 MHz is possibly very significant and probably responsible for the coupling loss. The frequency dependence is a giveaway. It\'s only a 1.25 dB deviation- which is not significant for RF.

 FT-50-43 50 to 1 transformer.
 The builder did show a 2 port VNA graph, it\'s flat, 100kHz to 30MHz,
but, I think he was driving a 50Ω on the 1 turn secondary,
I don\'t know what it would look like with 0.5Ω load.
                                              Mikek

 

[John Larkin]

On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote:

On 10/5/2022 10:15 PM, John Larkin wrote:
On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz,
on up to 7.70? at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

A fast opamp would be better.

Or a buffer, like BUF602. It\'s typically 1.4 ohms at 10 MHz.


If OP is committed to discretes there\'s always the classic White follower:

https://norcim-rc.club/Radio15_files/image025.jpg

Which should have substantially better output impedance at high frequency.

(You can make one of the output transistors PNP and add a third
transistor of either polarity and eliminate the coupling capacitor)

A q-meter doesn\'t need a radically low source impedance. Math it out.

 

[bitrex]

On 10/6/2022 3:34 PM, John Larkin wrote:

On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote:

On 10/5/2022 10:15 PM, John Larkin wrote:
On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz,
on up to 7.70? at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

A fast opamp would be better.

Or a buffer, like BUF602. It\'s typically 1.4 ohms at 10 MHz.


If OP is committed to discretes there\'s always the classic White follower:

https://norcim-rc.club/Radio15_files/image025.jpg

Which should have substantially better output impedance at high frequency.

(You can make one of the output transistors PNP and add a third
transistor of either polarity and eliminate the coupling capacitor)

A q-meter doesn\'t need a radically low source impedance. Math it out.

I meant better than the Darlington, I\'d be surprised if two transistors
could beat a BUF602 at 10MHz.

But I don\'t know what\'s in the BUF the datasheet doesn\'t seem to be
telling. 8000 V/us seems ludicrously fast, though

 

[amdx]

On 10/6/2022 2:34 PM, John Larkin wrote:

On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote:

On 10/5/2022 10:15 PM, John Larkin wrote:
On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0
One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz,
on up to 7.70? at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek
A fast opamp would be better.

Or a buffer, like BUF602. It\'s typically 1.4 ohms at 10 MHz.

If OP is committed to discretes there\'s always the classic White follower:

https://norcim-rc.club/Radio15_files/image025.jpg

Which should have substantially better output impedance at high frequency.

(You can make one of the output transistors PNP and add a third
transistor of either polarity and eliminate the coupling capacitor)
A q-meter doesn\'t need a radically low source impedance. Math it out.

 John that\'s true, but in a system where all you can know or measure is
the output of your signal generator,
a higher source impedance does introduce more error.
I probably need something clarified, on the HP4342A, they put a 75Ω
resistor across the transformer primary,
The design under discussion uses a 50Ω.   What is the purpose of that
resistor?
  With a 50Ω system, the error comes from the voltage divider 50Ω
source driving a parallel 50Ω//750Ω (=46.88Ω), where 750 is the
secondary load (0.3Ω) reflected back to the primary (50 to 1 turns
ratio). Vout = 2V x 46.88 / (50 + 46.88) = 46.88 / 96.88 = 0.968V for a
3.2% error.
  With a 2Ω source resistance and 600Ω across the transformer, the
error is 0.6%, but then if the source resistance rises to 5Ω,the error
grows to over 9%.
So if you have low source resistance, a constant value of source
resistance is important, otherwise, your better off with a 50Ω system.

                                                       Mikek

 

[piglet]

On 06/10/2022 00:51, amdx wrote:

Hi all,
  I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0

One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz,
on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
   I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
 Would like to see it lower, especially at he high frequencies.
                                                    Mikek

I quite like npn-pnp-npn triples for extreme followers. See fig 16 in
the ever great Rod Elliott page on followers:

<https://sound-au.com/articles/followers.html>

piglet

 

[Phil Hobbs]

piglet wrote:

On 06/10/2022 00:51, amdx wrote:
Hi all,
   I\'m following a group discussing building a Q meter. HP has a two
transistor Impedance Converter Ass\'y.
https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0

One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at
1MHz, on up to 7.70Ω at 30MHz after trying several
common transistors for the first transistor.
The output transistor is a 2n3866. The first transistor was a
2n4401,this gave the lowest output impedance.
    I\'m looking for some advice on transistors that will produce the
lowest output impedance in this circuit.
  Would like to see it lower, especially at he high frequencies.
                                                     Mikek


I quite like npn-pnp-npn triples for extreme followers. See fig 16 in
the ever great Rod Elliott page on followers:

https://sound-au.com/articles/followers.html

piglet

You can make that work if you take care that it doesn\'t oscillate.
Followers driving followers have a bad rep for that, unless (a) the
whole thing is running at sufficiently low current, or (b) there\'s a
base stopper on the output stage.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com