Re: Second Stage of Op-Am (Current to Voltage)

From: Robert Monsen (rcsurname_at_comcast.net)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 19:40:46 GMT

Monty Hall wrote:
> "Active8" <reply2group@ndbbm.net> wrote in message
>
>>I used to believe that the current would flow bidirectionally 'till
>>I wondered where I'd get that reverse current. You both are welcome
>>to simulate this circuit in Spice and try for yourselves.
>>
>
>
> I was looking to understand the op-amp analytically - not plug and chug root
> finding in SPICE. I want an articulation
> as to why it works - not "it just does" or "what else could it be...",
> "where else could it come from" - heard those a million times. Simply
> put... please prove it - analytically. So far it doesn't seem you're
> defending your position very well - or rather at all. That's why you're
> "repeating" yourself. "SPICE says Iout can't be negative" fine - then
> please explain/prove why (analytically using dc-analysis or small signal
> analysis) or point me to some text that can.
>

I believe that Iout can't be negative in the steady state because there
isn't a source of current other than leakage through the output
transistor. Current in the positive direction will flow through the
diode in the output transistor.

However, the current never has to go negative to control the output. It
simply has to control the positive Iout appropriately (which in turn
controls Vbe, in a way that depends on beta), and work that against Io.
The output can vary between -Vee + Vbe and Vcc simply by controlling the
base current.

>>>
>>
>>>> +Vcc +Vcc
>>>> o o
>>>> | |
>>>> | |
>>>> 2 Ic = Bias Current Io = Bias Current
>>>> | |
>>>> | |
>>>> | |
>>>> o------o-----o |
>>>> | | |
>>>> |< >| o-------o----o Vo
>>>> Vin(-) -| Q1 Q2 |- Vin(+) | |
>>>> |\ /| Cc --- o
>>>> |Ic1 Ic2 | --- |
>>>> | | Vo' | |/
>>>> | o----------------------o---o-|
>>>> | | Iout = Ic2 - Ic1 (+/- 2 Ic)|>
>>>> | | o
>>>> | | |
>>>> | | Ic1 |
>>>> | o |
>>>> | |/ |
>>>> o----o---o-| Q4 |
>>>> | | |> |
>>>> | | Ic1 o |
>>>> | o | |
>>>> | |/ | |
>>>> |--| Q3 | |
>>>> |> | |
>>>> | | |
>>>> -------o |
>>>> | |
>>>> | |
>>>> -Vee -Vee 

-- 
Regards,
   Robert Monsen
"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.
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