Power amp for function generator

Question

I have designed a function generator with the XR2206. That works well. I can take it up to 2MHz (sacrificing on THD, however).

Where I have failed so far, is the power stage. I have a supply of +10V/-10V for the power stage. I would want 1,5-2A max. output current up to 2MHz.

I tried a transistor output stage, but it falls apart beyond 300kHz. Means, the signal is getting worse and worse with higher frequencies.

I tried LM675 and it does its job well. Problem is, it requires a min. gain of 10 to be stable and with the 5,5MHz GBP the amplitude goes down beyond 500kHz. As I would like to keep the amplitude (gain) stable accross the freq. spectrum I added an LM7171 to boost the LM675. That works well, but with fixed gains for each stage, the amplitude is still going down beyond the 500kHz. Thus I tried feedback across both stages and then they became unstable.

So I hope, that you guys can help me, because I run out of ideas.

1. How would the schematic for both op-amps in series look like, given the requirement, that the total gain is stable up to 2MHz?
2. I could add an AB transistor stage to the LM7171 to get the output current. How would that look like? Or rather what transistors would you recommend, which have the necessary performance across the freq. range?
3. Any other suggestions are welcome.

Thanks in advance.

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I can ditch the LM675, but I should have a better replacement. I have looked for long and did not find a better replacement though. Either they don't have the power or the GBP is even lower. The solution might be the LM7171 + transistors. Question is which transistors will deliver the power at 2MHz? No, I don't have a simulator. Even with a simulator, I would prefer a schematic of an amp, which has been built up in real life before.

Answer 1

I solved the problem. I used the LM7171 to drive a MOSFET power stage. That works well to the 2MHz, even for square wave.

Answer 2

Bipolar power stages don't go much beyond 400khz. you'd need some kind of LFxx series transistors or mosfets, but you'll find mosfets have signal distortion in every cases if you don't power match them both on output and a pre-mosfet stage betwen preamp and the signal generator. That requires PFC inductor and current control for the mosfet stage and a few bandwidth corrections to get on the train.