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fully-differential amplifier with discrete devices

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Zeyneb:
Hi dear peoples,

I hope you like this post. For fun I would like to see if I could make a fully differential amplifier out of discrete devices. You know, one that also have an inverting output.

TI has an Application Report titled: fully-differential amplifiers SLOA054E. From this report I've the following simplified schematic:



I think it would be cool if I can realize this circuit with RF BJTs, ones that have multiple transistors in the same SMT package like the HFA3046BZ from Renesas for example.

However it has been a while when I was doing analog circuits so this might turn out to be a naive question.
In order to start simple I think it will be best to see if I can get something reasonable working in LTSpice with devices like the 2n2222 and 2n2907.

Ok I did watch the great tutorial by w2aew: #193: Back to Basics: the differential amplifier, aka long-tailed pair, diff-pair

And put in LTSpice the following circuit:



But with this circuit I loose the voltage swing of Qn. So then I tried the following circuit:



But now node Q as well as Qn stay slightly below 9V supply. As Q4 and Q5 are in saturation. I couldn't nail down where the problem lies.

Questions
1) Are there more detailed resources to teach me to build the fully-differential amplifier with discrete devices?
2) How can I change my circuit to make progress getting the FDA to operate?

Best regards,
Zeyneb

T3sl4co1l:
Right, you can't bias a fixed current source with a fixed current source.  This only works closed loop -- you need to add the Vcm / bias servo circuit. :)

You're also trying to measure the diff amp itself in open loop, which is sure to saturate from the slightest perturbation.  You can set DC-op using a huge inductor and capacitor, to close the loop at DC but leave it open at any meaningful AC frequency.

Tim

Zeyneb:
Hi Tim,

Thanks for your suggestions. Can you please provide a link where I can learn about Vcm / bias servo circuit?

Will the Vcm / bias servo circuit close the loop with negative feedback? So that I can further experiment with the circuit. Or do I need to add that as well. Like Rf and Rg in the TI Application Report.

Ok but you think my network of pnp transistors is not wrong? As you didn't say something about that?

Thanks
Regards Zeyneb

T3sl4co1l:
You need the two R's and Vocm Error Amp, which will control the current through R1, say by lifting one end from ground and driving it with the error amp output instead.

And probably make R1 half the... wait, R1, R3 and R7 are all the wrong values anyway, as shown you've got ~0.62mA tail current but 5.5mA collector current.  Of course the collectors will always be saturated to Vcc...

R7 = 3.6k, R3 = 680 and R1 = 4.16k would be typical, with R1 driven by an error amp.

Note that putting the CM sense R's on the collectors directly, will significantly reduce the differential mode gain.  You really need the buffers in there to get it back.  Just for setting up the CM loop, it's fine.

You can use a virtual (5 terminal) op-amp for the Vocm EA, or construct one from discretes if you like (but test it independently first, so you know you've got the voltage range and biasing correct!).

Tim

Zeyneb:
Hi Tim,

I hope I do understand you. The two R's you mentioned are taken from this 1st stage Q and Qn?

If I fully make the rest of the circuit as shown by TI I would need a second Vocm error amplifier, with buffers?

Thanks a lot for your time!!

Zeyneb

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