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Design >> Analog Design >> Why cant an OTA drive resistive loads..?
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Message started by sylak on May 24^th, 2006, 4:44pm

Title: Why cant an OTA drive resistive loads..?
Post by sylak on May 24^th, 2006, 4:44pm

It is usually said that opamps are used for resistive loads and OTAs for capactive loads...

Why is that so...?

Title: Re: Why cant an OTA drive resistive loads..?
Post by uncle_ezra on May 24^th, 2006, 5:25pm

Well if OTAs were to drive resistive load its gain would depend on its output impedance in parallel with the resistive load. So a low resistive load would affect the gain, so usually you drive resistive load with a low impedance output.

Title: Re: Why cant an OTA drive resistive loads..?
Post by schehrazi on May 25^th, 2006, 2:48pm

Another way to look at it: OTA is an Operational Transconductance Amplifier which means it converts voltage to current. Therefore, the output port is ideally a voltage controlled current source. Now, in a real circuit, there is some output impedance in parallel with that current source, the resistive part of which should be very big so that the DC gain is very large. We need this large output resistance so that the output port of the OTA can be approximated with an ideal current source. If you connect a small resistive load (compared to the output resistance) to an OTA, its DC gain drops. An OpAmp is basically an OTA followed by a driver stage, like a source follower, which reduces the output resistance and can drive small loads.

Title: Re: Why cant an OTA drive resistive loads..?
Post by Raul on May 30^th, 2006, 3:52pm

you can drive a resistor with an OTA, it's just that your open loop gain is going to be directly determined by the load resistor, avol=gmi/gload. if you don't care about this last thing them...