DIY Stompboxes > Building your own stompbox

Q for RG ... hfe/gain in buffers

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RedHouse:
RG,

Reading recently one of your posts you mention the MPS-A18 as being the lowest noise transistor (US made) and so looking at the transistor data sheet it says it hfe is like 500->1500, that's some range.

Anyway my question is regarding buffers and the hfe of the transistor used, does the hfe set the gain of the bufer?

For instance take JD's circuit here:


if I used the 220K/220k/3.3k resistors and an MPS-A18 for Q1 would it be a buffer or would I be getting the huge 500->1500 gains the hfe spec indicates from it?

My goal is finding a minimal gain buffer to use as a building block when isolating two circuits, say a output buffer for a classic Wah or FF or something, hence the need for low noise.
(I'm not understanding the hfe/gain relationship I guess)

puretube:
that emitter-follower circuit always has a gain of ~1,
regardless of trans.-type.
(someone else may explain more into detail...)

petemoore:
I don't even socket buffers transistors anymore. One can stick just about anything in there that's NPN and it will work, some have lower noise though.
  I usually just grab a 2N5089 for buffers, and solder it right in./
  Could be I'm missing something, but they drive just fine.

R.G.:
Follower circuits by their nature try to but never achieve a gain of 1.

Emitter-, source-, and cathode-followers all employ 100% feedback to do their work. They do this by means of their transconductance - the amount of current that flows in the output as as result of a change in the input voltage. The output current is converted to the output voltage by flowing through a resistor.

The transconductance is literally the change in output current with a change in input voltage. For FETs and tubes, the transconductance is a specified quantity on the data sheet, for bipolars, it is usually not.

Tubes have the lowest transconductance of the bunch; a tube cathode follower will have a "gain" of about 0.90 to 0.95. This is because the output current changes little (on an absolute basis) for each volt of input voltage change.

JFETs are better, and will get into the higher 0.9's. MOSFETs have a big transconductance, usually about 1A/V, and so they do a much better job.

Bipolars are usually the winner in this contest, because they have a transconductance typically 10x that of MOSFETs. Only a tiny change in base voltage produces major changes in output current. In fact, the change needed is so small that to a first approximation we think of bipolar base-emitter voltage as constant at a fraction of a volt. The gain of a bipolar emitter follower is typically 0.99 for low gain devices, and it can get much better. It's best thought of as a gain of one minus an eyelash.

Even within bipolars, the higher the current gain, the closer the output gain is to one.

But your question is about noise.

Noise comes from a couple of places in a follower. It comes from thermal noise in the follower resistor, it comes from the thermal and excess noise in the transistor, and it comes from the thermal noise in the biasing arrangement. The good news is that since the gain is so low, you usually can ignore these sources. The only time that is not true is when the transistor is noisy. Most modern transistors are very good, and the 2N5088, 2N5089, and MPSA18 are exceptionally good. Just don't ever reverse-break the base-emitter junction and they'll stay that way.

So use any one of those with impunity as your buffer. They will not be where the noise comes from.

RedHouse:
Thanks RG, always the gentleman!

Also thank you puretube and petemoore for posting on this.

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