From: Winfield Hill
Subject: Re: Transistor amplifiers v. frequency in SPICE
Date: 13 Jan 2003 02:58:34 -0800
Organization: Rowland Institute
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> Winfield Hill wrote:
>> John wrote...
>>> Winfield Hill wrote ...
>>>> +24V | 1nF
>>>> | +----||--- out
>>>> 150k |
>>>> 1nF | C BC337
>>>> --||--+---B 2n2222
>>>> | E
>>>> 150k |
>>>> | +-----,
>>>> gnd | |
>>>> 140k 1nF
>>>> | |
>>>> gnd gnd
>>>> Another problem with your circuit (did I get it right?),
>>> I couldn't make anything else out of it. But the OP claimed 1 V
>>> out for 2 mV in at low frequencies. At 80 uA, re = 490 ohms, so
>>> he's trying to get a gain of 300 out of one stage, assuming that
>>> that the load impedance is high compared with 150 kohms, which
>>> it won't be, of course.
>> It'll be infinite in spice. :>) At low frequencies the expected
>> gain could be 150k/re = 150k/312 = 480 (note, 25/0.08 = 312), so
>> yes, 300 is reasonable, assuming an infinite transistor collector
>> resistance (a very high Early voltage), which isn't correct, IIRC.
> At the low frequency mentioned by the OP (100KHz) the emitter bypass
> is much higher impedance than re and will therefore reduce the gain
> to considerably less than 300, nearer 80 or so.
Good point Rick. The design has so many problems, it's a
possible candidate for our "Bad Circuits" section. :>)