Darlington pair OTA output

[juz_ad]

I have an OTA with an op-amp I-V converter giving output at unity gain on a 20Hz +/- 5V sine wave input. It works fine, basic circuit is below:



I wanted to swap the op amp I-V converter for a transistor output - so I simulated a darlington pair in isolation and was able to reproduce the same output as the I-V converter (although the darlington is inverted - which is fine).



When I added the darlington pair to the original OTA - you can see the output is lower amplitude, clipping and doesn't want to swing below approx. -2V - regardless of the bias R values.

[Edit: with values for R17 < approx. 5K the output will swing below -2V but I'm still losing the gain. Thanks.]



Can anyone suggest a possible cause and solution to this? Thanks!

 

[godfreyl]

Hi
The problem with the third circuit is that the Darlington pair has unity voltage gain, the same as a simple emitter follower. So the voltage swing at the output is about the same as the voltage swing at the collectors of Q2 and Q4, which is obviously limited.

It's better to use a PNP common emitter stage, as in the pic below. This allows a large output voltage swing, with very little voltage swing at the collectors of Q2 and Q4.