Re: Need to drop 0.2 V with 2-ternimal series device
From: Winfield Hill (hill_a_at_t_rowland-dotties-harvard-dot.s-edu)
Date: 03/18/05
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Date: 18 Mar 2005 05:33:21 -0800
lemonjuice wrote...
>
> Winfield Hill wrote:
>
>> lemonjuice wrote...
>>>
>>> BTW The circuit would have a worse problem. you've got a saturation
>>> conditions at the opamp inputs. V(-)= 1.434V and V(+)= 0.115V.
>>
>> I'm not sure where you get those numbers, or what point you're
>> trying to make, but I'll say this, sub-100mV collector-saturation
>> voltages for sub-100uA currents (i.e., Rsat under 1k) is entirely
>> reasonable for some low-power opamp output stages.
> For example
> According to the zener specs it should drop 1.25V. That would mean
> .3 and 1.35 between the output and the V(+) node , across the 68K
> resistor. Meaning current through resistor is 1.05/60.8K which
> doesn't add up to whats written there.
You seem rather badly confused, there's 100mV across the resistor
when the circuit is at equilibrium. The circuit is an output-voltage
regulator, not a fixed-voltage-dropper, as the OP originally proposed,
perhaps that explains your confusion?
> I tried a Microcap simulation and i got V(-) almost equal to the
> supply voltage, which is more or less what I'd expect from Positive
> feedback increasing to infinite the output impedance which somewhat
> agrees with my earlier calculation.
The overall feedback is negative, not positive at equilibrium. That's
because the zener dynamic impedance is much much lower than 68k. The
output impedance is certainly NOT infinite. I'm going to sign off.
--
Thanks,
- Win
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