[sdiy] CEM3372 Notes

ASSI Stromeko at nexgo.de
Wed Jun 19 20:10:47 CEST 2013

On Tuesday 18 June 2013, 16:51:36, Russell McClellan wrote:
> I tried to do the analysis in a way someone with no OTA experience
> could follow.  If I were assuming familiarity with OTAs, I would have
> said something like "It's an OTA, but the final two current mirrors
> are removed, and the output current is connected directly to the
> inverting input, with a capacitor to ground forming an RC filter with
> the input impedence of the differential stage."

I guess you could say that, but that circuit would normally be recognized as 
a differential pair with active load and a single-ended output, a standard 
input stage in most operational amplifiers.  Here, the second stage is 
missing and local feedback is applied to create a unity-gain voltage 
follower.  It only works as a filter because a) the output impedance of that 
OPA is rather high and b) it can be conveniently controlled with the tail 
current.  This together with the capacitive load creates the lowpass 

> I'm hoping for some constructive feedback as I'm quite aware my
> technical writing skills, and my analog chops, are fairly poor.

As a general remark, don't draw diode-connected transistors as simple pn-
diodes in your schematics.  There is a difference and it will make itself 
known when you try to simulate the circuit.

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