[sdiy] Why do we need the buffers? - gm-C filter question

[René Schmitz]

Hi Aaron,

> Conjectures:
> 
> 1) Maybe the OTAs we typically use, 3080, 13700, discrete 4-BJT, 
> whatever, don't have high enough input impedance to avoid having the 
> next stage load down the previous one? (One thing that makes me think 
> this is the IC books are all CMOS, and a CMOS-based OTA would probably 
> have higher input impedance, so maybe they could get away with it)

Your BJT OTA inputs aren't really hi Z in the same sense that opamp 
inputs are.
Basically the input current is a function of the input differential pair 
transistors beta and the amplifier control current Iabc. Ib=Ic/ß. ->
So assuming a beta of say 200 and 2*Ic = Iabc = 0.5mA, you'll get 
12.5uA... A lot more than the nA or even pA we see with bipolar or JFET 
and MOSFET opamps. Worse still that it shifts with Iabc.

> 2) To keep the input voltage on the next stage low enough to avoid 
> distortion, you'd have to keep the current coming out of the previous 
> stage pretty small; maybe this gets into noise issues (although 
> constantly running through all those resistive dividers and op-amp 
> stages can't be good), and also, it may severely limit the range of 
> useful currents at the OTA input pin.

I think you're on the right track here. If you consider that the input 
current is in the area of micro amps already, you'll loose a lot of 
control range this way, since a lot of that current tends to get sucked 
away into the next stage.

> Am I right (or partially right and partially wrong) on (1) and/or (2)? 
> Other reasons?

But a collorary question could be to ask, why not to use buffers, and 
just divide down at the input and then boost back at the output. Running 
all stages in between at the same voltage level.
I'd say the answer to that is its been traditional folklore to do it the 
other way. You know, an engineer finds this one pole circuit on the 
datasheet. Cut and paste 4 times. :-)

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159