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

From: Active8 (reply2group_at_ndbbm.net)
Date: 12/29/04 

Date: Tue, 28 Dec 2004 20:38:04 -0500

On Tue, 28 Dec 2004 16:27:48 -0800, Larry Brasfield wrote:

> "Active8" <reply2group@ndbbm.net> wrote in message news:1zj2et2f2kpc$.dlg@news.individual.net...
>> On Tue, 28 Dec 2004 13:26:01 -0800, Larry Brasfield wrote:
>>
>>> "Active8" <reply2group@ndbbm.net> wrote in message news:1xevajibb64in.dlg@news.individual.net...
>>>> On Tue, 28 Dec 2004 12:09:45 -0800, Larry Brasfield wrote:
>>>>
>>>>> "Active8" <reply2group@ndbbm.net> wrote in message news:9d5t5ofpc4fj.dlg@news.individual.net...
>>>>>> On Tue, 28 Dec 2004 05:12:04 -0800, Larry Brasfield wrote:
> ...
>>>> No I haven't. See the Q4 impedance term in || with the other loads?
>>>
>>> I do and did see it. It is a minor term.
>>
>> Bull! It's a gain limiting term.
>
> Let's see ... You have Ro_Q4 in parallel with Ri_Qmiller.
> Ri_Qmiller is going to be in the neighborhood of
> Beta * (Vt / Io)
> whereas ro_Q4 will be something like
> Vearly / Ic1
> So, for Ri_Qmiller to be comparable to Ro_Q4, (and taking
> Io to be similar to Ic1, an assumption generous to you), we
> need to have Beta * Vt =~ Vearly or Beta =~ Vearly / Vt.
> Typical values of Vearly are 40, and Vt is about 26 mV,
> so you need Beta near 1500 in order to keep the Q4
> impedance term from becoming minor. It gets worse with
> a more reasonable assumption about Io relative to Ic1.
>
> Perhaps you have overlooked the fact that the smaller
> impedances dominate in a parallel connection.

Nope and if you were following the threads, you'd know that the
circuit OP linked to has a follower *and* a CE stage for the miller
stage with a 100 ohm resistor on the emitter of the CE stage. So if
beta were 100 that's Rin of 1 Meg. Sorry, I never shifted gears back
to the figure below. Big difference.
>
>>>> ROFLMAO. When Vo is mid-rail, currnt flows to the right from the Vo'
>>>> node. So even if Q4 was in cutoff (and it isn't), it wouldn't
>>>> matter. The current would come from Q2.
>>>
>>> In that state, Q2 and Q4 collector currents are nearling balanced.
>>> To say the output comes from one only is oversimplified. Once
>>> at that state, if Vo begins to move positive at a rate faster
>>> than Ib(Qnameless)/Cc, net current at Vo' will be leaving
>>> to the left. According to your way of thinking, it would be
>>> going to Q4.
>>
>> That's differen't from what the OP was asking (look at the OP) and
>> as I said, it's an effect of Cc - that may or may not manifest, BTW,
>> like you said. You mean slewing. That's not the same as the
>> condition given i.e., Ic1 - Ic2 = Iout negative.
>
> Do you believe that Ic1 can never exceed Ic2? If so,
> what do you suppose happens if (Vin(+) - Vin(-))
> is more positive than, say, a few 10's of mV?
> If not, why can't you see that since Iout is defined
> as Ic2-Ic1, that expression can become negative?

So what the expression can be negative? Leave it there until the cap
is done discharging and tell me where the current will come from.

> And please, no jabber about the steady state. We
> are adressing your proposition, and I quote: "I told
> you Iout can't be negative" and "I used to believe
> that the current would flow bidirectionally".

I still do as long as we're not trying to get bidirectional flow
from the base of a bjt. If the cap (even the b-c cap) pumps current
that way sometimes, that's a transient response. After that, the
diff stage can ask for as much as it wants and if it could go below
the rail, it'd keep doing so until the b-e breaks down in reverse.
>
>>
>>>
>>>>>>>> +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
>>>>>>>> (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
>
> This is getting a bit wearying, so I may not respond further. 

-- 
Best Regards,
Mike
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