differential mode output from opamp with current mirror load

[amic]

Current mirror load convert differential currents of input pair into single ended output , If I am not wrong…but I was curious if we can still take differential output from such a structure ?? anybody can throw some light on this ….

 

[ptmsl]

Re: differential mode output from opamp with current mirror

If you use a current mirror as a load for a long tail pair then the differential nature of the output is lost. If you need differential output you can use resistors as load or use active loads (current sources) if you need very high gain but with the risk of high offset. There are opamps that have a differential second stage. They use active loads and the input of the second differential amplifier (second long tail pair) as a load for the differential current output of the first stage. For an example look at the OPA642/3 opamps from Burr-Brown(Texas) or the LF356 from National

 

[rockycheng]

Re: differential mode output from opamp with current mirror

Current mirror load convert differential currents of input pair into single ended output , If I am not wrong…but I was curious if we can still take differential output from such a structure ?? anybody can throw some light on this ….

You cannot call the load of differential output opamp "current mirror load", but CURRENT SOURCE LOAD instead.

 

[bageduke]

Re: differential mode output from opamp with current mirror

The main point is - If you check two output resistance of differential pair with current mirror, one is 1/gm, another is rds||rds - which will lose symmetrical feature. The better way is to use both diode connected MOS as load, or both biased current load. Now transconductance of each side is 0.5gm.

For diff-in diff-out opamp, you have to consider the circuit stablization issue of common-mode output voltage.