OTA Amplifier and Voltage Amplfier Comparison

ota amplifier

Guys,

Need some feedback on the OTA and Voltage amplifier. As we know that Voltage amplifer will have low output impedance and we can say it sense input and output as voltage to voltage whereby OTA will have high output impdeance and will sense voltage to current which the gain is transconductance. Both amplifer can be applied input as voltage signals and output as voltage signal as well. So, what is the solid different between them. If you say OTA behave as current source due to high output impedance, then where is the current sourced to as even Voltage Amplifier also source current at the output to charge/discharge the load caps.

Regards,
Suria

ota as a constant current source

I will try to answer as best as I could. A voltage amplifier is just an OTA with a buffer output stage. As such, a voltage amplifier can drive resistive load. The current needed to drive the resistive load is sourcing from the ouput buffer stage such that the gain of the amplifier is not affected. On the other hand, an OTA is not suitable for driving resistive load because any current needed to drive the load will have to come from the output stage of the OTA. Subsequently, the performance of the OTA will be affected. But OTA will be suitable to drive capacitive load. Hope that clears your doubt.

comparison of different otas

In case of OTA error current (difference in source and sink current ) due to input difference i.e., vinp-vinn either charges or discharges the capacitor .So OTA output is error current which charges /discharges the capacitor to ensure that input differnce is small .

In amplifer which is OTA + Buffer ,if we consider buffer as a source follower then OTA sets the gate cap of source follower to desired voltage by its error current and the gate voltage is reflected at the source (output of amplifier ) which is ready to drive resisitive loads .

think that in an OTA either source current and sink current change based on the input differnce giving a charge/dicharge phenomenon at output cap , but in a two stage amplifier either source or sink current is always a constant since it is mirror of tail transistor .

Hope this helps ...

drive resistive load amplifier

pseudockb said:

I will try to answer as best as I could. A voltage amplifier is just an OTA with a buffer output stage. As such, a voltage amplifier can drive resistive load. The current needed to drive the resistive load is sourcing from the ouput buffer stage such that the gain of the amplifier is not affected. On the other hand, an OTA is not suitable for driving resistive load because any current needed to drive the load will have to come from the output stage of the OTA. Subsequently, the performance of the OTA will be affected. But OTA will be suitable to drive capacitive load. Hope that clears your doubt.

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Thanks, at least to me, it is a good explaination

Scottie