[sdiy] Buchla 194 Fixed Bandpass Filter analysis

[Aaron Lanterman]

On Apr 17, 2008, at 2:21 AM, Donald Tillman wrote:

>> 1) The highpass S-K has a "feedback resistor" and a "resistor to
>> ground." When I'm considering the "resistor to ground," should I
>> think of the 68K and 100K biasing setup as both being resistors
>> in parallel to "AC ground?" If so, my "to ground" resistor is
>> actually 68K || 100K = 40K.
>
> To AC and DC ground, yeah.  It's called a Thevenin equivalent
> (Wikipedia has an entry for Thevenin's Theorem).  A resistor divider
> between two voltage sources such as the power rails is equivalent to a
> single resistor, of value equal to the parallel combination of the
> two, wired to a single voltage in between.

I'm down with Thevenin. I just always get weirded out when I see a  
transistor. ;)

>> 2) All of the bases of the voltage buffers, except for the first one,
>> have 10K resistors going into the bases. Why are those 10K resistors
>> there?
>
> Discrete emitter followers often include base resistors to hold down
> high frequency oscillations.  Similar to the dominant pole in opamp
> circuits.

Thanks!

>> 3) There's a 2K2 and 100K voltage divider right at the beginning
>> that attenuates the signal by a factor of 50, presumably to get
>> the signal into the "linear small signal analysis"
>> range. But... I don't see anywhere where the gain is brought back
>> up at the end! Is there something I'm missing?
>
> The resistor values are actually the other way around; the circuit
> only attenutates the input signal by 1/50th.  The 100K resistor keeps
> the input cap from floating and prevents loud pops when you connect a
> ground-centered input, and the 2.2K protects the transistor from nasty
> hight current transients.

Bah, I'm an idiot... I've been looking at so many OTA circuits that I  
flipped that around in my head!

- Aaron