[sdiy] CEM3372 passband gain trick?

[Magnus Danielson]

On 08/13/2012 10:13 PM, Dave Manley wrote:
> The mini filter is discrete, so adding an opamp is easiest,  although it would interesting (perverse?) to keep it discrete.

Keep your perversities to yourself and your MiniMoog. :)

It's a two-tree transistors away, maybe four.

A diff-pair (like the bottom of your Moog chain), with a resistor at the 
top of the collectors on both sides, then a current mirror on the common 
emitter side, and then a resistor from +15V to give the current for the 
current mirror. The bases is the + and - inputs. There, done it. 
Counting in 4 NPN transistors and three resistors. Keep the current to 
around 1 mA to start with, so the current resistor should be 15 kOhm. 
Start with 6k8 resistors on each arm and see how that goes. The current 
mirror has the collector and base connected to the 15k resistor while 
the emittor hits the ground rail, the other transistor has it's emitter 
on the ground rail, the base to the first transistors base (and 
collector) and then the collector hit's the transistor pairs common 
emitters.

This is a febrish head's back of the envelope sketch (without the 
envelope!), so it hasn't been proven in battle. Using a current mirror 
will improve performance to "just" using a resistor, for the cost of two 
transistors. One of them may actually be replaced by a

Cheers,
Magnus