NSFW What's up with the buffers in cornish designs?

[giovanni]

No sweat man. It was fresh in my mind because I was working on it today and this seemed the most logical place to post it.

What I actually found interesting were a few things most of us would see as “kludges” or “fudging it”.

R1 and R8 come to mind. Obviously Cornish was targeting a specific input impedance he couldn't get with the bootstrapped emitter follower alone.

It seems that emitter voltage around 50% of source voltage was important to him, so R1 acting as an artificial reducer of input impedance made more sense than trying to get blood from a stone.

Furthermore, it seems that 100 ohm output impedance was a design criteria so he simple added R8 in series to get there.

If anything, I appreciate him a bit more because of these little practical things.

What do you mean by source voltage?

 

[Big Monk]

What do you mean by source voltage?

For the purposes of calculations, I assume a raw source voltage, in this case 9v.

In practical terms it would be the whatever the filtered source voltage is. Vf is usually what I call it in my schematics.

After the diode and filter caps.

 

[giovanni]

For the purposes of calculations, I assume a raw source voltage, in this case 9v.

In practical terms it would be the whatever the filtered source voltage is. Vf is usually what I call it in my schematics.

After the diode and filter caps.

Ah ok. Source is a bit of an overloaded term given that it indicates one of the pins of a FET. I think it’s relatively common to want the emitter voltage at half Vcc because that maximizes the dynamic range (from 0 to Vcc) of the output signal before clipping.