DIY Stompboxes > Building your own stompbox

Q for RG ... hfe/gain in buffers

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Would the use of a darlington in this circuit (MPS-A13) get the gain up to 1?

Dunlop in the newer Crybaby circuits, use an MPSA13 as an input buffer, could this possibly be why.

the MPSA is probably rather there for the high input-impedance (?)

Joe Davisson:
For non-inverting voltage gain you can use a common-base amp, but the input sensitivity is kinda low. Otherwise it will work for audio, as long as the desired gain is pretty low (like 1x-2x).

Example (2x gain):

Base: 10k/10k divider.
Collector: 20k to 9v.
Emitter: 20k to ground.

Input into emitter, output from collector. You can get some (unpleasent?) sounds by using high resistances. Driving it directly with an emitter-follower corrects the impedance, giving you a non-inverting buffer with voltage gain =) Great until it clips =(



--- Quote ---Would the use of a darlington in this circuit (MPS-A13) get the gain up to 1?
--- End quote ---

No. The math works out that the gain of a simple emitter follower approaches 1 asymptotically - that is, it gets infinitely close to 1 as current gain increases, but never actually gets there. A darlington gets insignificantly nearer 1 than a for instance 500hfe ordinary BJT.

--- Quote ---the MPSA is probably rather there for the high input-impedance (?)
--- End quote ---

Yes. Using a very high gain transistor (MPSA18 at 500 - 1200) or darlington (1000 to 50,000) lets you get follower action with a vanishingly small input base current, so you can make the biasing resistors much larger. A darlington lets you get into the 1M region with biasing resistors, so the input of the buffer is much higher impedance, and sucks tone less there.

However, you can easily add noise with darlingtons if you're not careful. Darlingtons invite the casual designer to bias them with a simple resistor string from + to ground connected to the base. This happens to *maximize* the resistors' noise into the base, especially with high value resistors. Using two lower value resistors, say about 10K, to set the voltage and a third high value resistor to the base puts a very small voltage across the impedance setting resistor at the base and lets you use a capacitor from the junction of the three resistors to ground to make the bias voltage quiet. This is known in some circles as "noiseless" biasing. It's worth remembering any time you use high gain bipolars or darlingtons, or any time you feel the urge to reach for a 1M.

Thanks RG, and thank you too Joe.

This is great stuff, you (I) just don't get this depth of info from books.


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