Instability on a diferential output amplifier

[Miguel Lopez]

Hello

I've been trying a diferential output amplifier, as it is my intention in the future to build a transistor oscilloscope, with better performance that the valve one that I'm currently using. Althought the circuit that I've been testing amplifies the input signal very well, it suffers from instability with no signal applied.

That means that with the input connected to ground, the voltage across points A and B (see picture) is constantly "moving". The range in which it moves, is about 0,6V. Then the trace on the scope will be moving too.

Voltage at points C and D, are very stable, and Vcc (60V) is also very stable. So I can not find tha reason for this instability.

I'm asking for help to solve such instability.

Thanks in advance

[orbanp1]

Hi Miguel,

A quick answer/guess to your question could be a non-perfect pot in the source of the FET.

Just curious, where is that circuit coming from? Is it your own development?

Designing a vertical amp for a scope is not a trivial task.
I guess one would start from the requirements...

Designing an amp to your own needs could be tremendous fun and very rewarding (assuming it works as expected). The suggestion would be to simulate the performance of the circuit first and only proceed with the implementation when the simulation results are satisfactory.

Also, you do not need to reinvent the wheel, so you should look at the schematics of transistorized scope vertical amps.
Vertical amps of transistorized scopes with about 5MHz to 10MHz bandwidth are usually a lot more complex than the circuit given above.
First comment would be on the role of the R11 pot in the source of Q5. It would both change the gain as well as the DC balance of the circuit! You really need to separate those two functions!

The Radiotechnika Evkonyve 1976 issue, pp. 33-69, (a yearly publication) had a long article on building scopes. It is in Hungarian, but it is still worth to look at the schematics.
(The author worked as a designer at EMG, at that time one of the biggest test equipment producers in the Eastern-block.)
It can be downloaded from here:
http://www.diagram.com.ua/english/li...nyve-magazine/

Good luck, Peter

[Radio Wrangler]

With no emitter resistors in the long-tail pair, you have it set to give high gain all in one stage. In a scope Y amplifier, I'd expect to see multiple differential amplifier stages, each with emitter degeneration, possibly with a cascode arrangement in the collectors, and with R-C compensation networks between the emitters to get flat gain across the frequency band.
The instability could be thermal drift in the transistors magnified by the high gain.

David

[Restoration73]

I see D3 is not decoupled for noise - might be worth a try.

[n_r_muir]

Hi Miguel

The Telequipment S54 has a relatively simple Y amplifier
http://elektrotanya.com/telequipment.../download.html

[MotorBikeLes]

That download has both S54 and S54A manuals. Whilst the S54 looks simpler than the S54A, its front end has two nuvistors.
About 30 years ago, being short of 13CW4 nuvistors, and having a few S54 scopes, I played about replacing the nuvistors with FETs. From memory, I simply put a zener (plus resistor maybe) in the supply to the anodes and a small electrolytic to 0v rail. Worked perfectly. I tried to do the same with a dynamco Y amp, but failed, I think because of characteristics of the Dynamco's nuvistors and my available FETs.
Les

[Miguel Lopez]

Originally Posted by orbanp1

you do not need to reinvent the wheel

The fact that I'm trying to build a scope at home, it is re-inventing the wheel by itself.

Originally Posted by orbanp1

where is that circuit coming from? Is it your own development?

I designed it, trying to use what I have at hand.

Originally Posted by orbanp1

you should look at the schematics of transistorized scope vertical amps.

I usually do it but they are usually very complex. And usually, I do not have the required components to build them.

Originally Posted by orbanp1

scopes with about 5MHz to 10MHz bandwidth

If I get 100kHz bandwidth, I will be very happy.

Originally Posted by orbanp1

First comment would be on the role of the R11 pot in the source of Q5. It would both change the gain as well as the DC balance of the circuit! You really need to separate those two functions!

I know I have to separate those functions, but I have to go step-by-step. One problem at the time. Thanks for your comments. I'm trying to download the magazine right now.