Help to understand "current drive"

[b_force]

You have Forum experience enough to know that subjects wander allover the place, here and in any other thread

That usually happens after a page or three.

It often feels like old grannies coffee or tea corner.
Just ignore any real information and just keep chit chatting for the hell of it.
(and then people wonder how to learn more .........)

 

[Baffless]

Thank you, Bernd. You are right, of course. But these emitter resistors don't introduce any loss over and above those in a conventional amplifier, and in any case, they are of a very low value (0.1 Ohms). But yes, we can call it mixed feedback. And I should add, the load can be grounded, and bridge-working is very straightforward.

But the opamp controls the global feedback taken from output over R14/R8. I see only voltage amp.

 

[Jontie]

If you read my paper, you will see that this is not just a voltage amp. In the example given, the output impedance is approx. R14/1000, so that if R14 were increased to 4 K Ohms, the output impedance would be 4 Ohms (I have measured this, and it is so). In the limit, when R14 approaches infinity, the amplifier becomes a trans-conductance device, giving a true current output.

John.

 

[Freedom666]

Do you sell this amplifier?

 

[Jontie]

No, I'm afraid not. I'm in my eighties now, and I don't want the hassle.

 

[Baffless]

If you read my paper, you will see that this is not just a voltage amp. In the example given, the output impedance is approx. R14/1000, so that if R14 were increased to 4 K Ohms, the output impedance would be 4 Ohms (I have measured this, and it is so). In the limit, when R14 approaches infinity, the amplifier becomes a trans-conductance device, giving a true current output.

John.

Now i see. So if no R14 is there, the stage after opamp should be pretty linear as opamp don’t provide any correction.

 

[Jontie]

That's right. The op-amp just defines the current through R8. The following stage is, in fact, very linear, provided attention is paid to matching the two halves.

 

[kgrlee]

In the limit, when R14 approaches infinity, the amplifier becomes a trans-conductance device, giving a true current output.

Have you tried this and measured the performance and output Z ?

 

[Jontie]

Yes, I have. I have built 12 of these amplifiers (not simulations, but real hardware) and the performance figures are shown in my original paper, and also in my post: "Amplifier with Predictable Output Impedance". The output Z is easily measured by changing the load resistance from (say) 8 Ohms to 4 ohms, and measuring the drop in output voltage.

​

 

[kgrlee]

What is the highest Zo you have built & measured with your circuit?
Your very impressive paper shows a configuration with Zo not too different from a valve amp.

 

[Jontie]

I have used Zo=8 Ohms in the past. Probably up to 1 K is possible with a little care. The example in the paper represents the lowest practical value (about 0.5 Ohms), due to power dissipation limits in the feedback resistor.