quick emitter follower question

The transistor adds noise to any signal applied to it. That noise does not get voltage amplified by the follower, but it still gets added to the signal.

OK, that's kind of what I thought too, but I thought I should make sure before choosing a suitable transistor.

Thanks John.

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Not only will the transistor add some noise, it will also add a lot of distortion. around 1% for 600mV peak to peak. The distortion raises proportional to the input level and is independent from the bias point. The noise is completely irrelevant compared to this phenomenon. If you want the signal "untouched", you should use an opamp as buffer, OP27 comes to mind.

There certainly will be noise added but not a lot.

Graham

What's the mechanism? With enough DC voltage across the emitter resistor and a light load, one can approach constant emitter current. All that's left is Early voltage effects, pretty small usually.

John

And of course, the real issue is the application. One worries about noise with weak signals, with larger signals the signal swamps out the noise.

A lot of places where emitter followers are used, the issue of noise will never come up.

On the other hand, one reason you see transformers in low level audio signals even today is because they are at a point where the signal is weak, and introducting noise at that point is not a good thing.

So they have those matching transformers between microphones and the preamp, and between those moving coil phono cartridges and the phono preamp, since they need to step up the voltage at those points without adding any noise. Once the signal is stepped up by the transformer, the noise of the following preamp is less important.

Michael

Thanks for all the replies.

Michael, since you mentioned the application.....

I currently have a really slick switching system for my guitar effects pedals. To make a long story short, the only problem with it is that it uses relays to switch the effects in and out, and they give an audible "pop" when switched. It's not horrible, but I was trying to find a way to make things cleaner (I can't leave well enough alone). I first thought of using a simple transistor or JFET as a switch in place of the relay, but a friend informed me that it wasn't possible to have the audio pass through the transistor if it was also being used as a switch (or is there a way?). We came up with the idea of using a transistor as an amp with very little gain and switching the amp on and off instead of a relay. Which brings us to this discussion.

Any thoughts on this line of reasoning?

Thanks

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Never add an open, simple emitter follower to a high quality audio circuit. As was mentioned they will add distortion to the signal which may be tolerable in a phone or other circuit where high quality audio is not required. This happens because the internal emitter resistance, re, is a function of the emitter current which in turn is a function of the base to emitter voltage. This makes a variable, non-linear, voltage divider with the load impedance. The nominal gain is one, but in reality is always less than one and varies with the voltage level. That non-linearity causes excessive distortion both harmonic and intermodulation.

Emitter followers are used inside vitually all bi-polar amplifiers, op-amps, etc. But, they are ALWAYS inside feed back loops attended with current sources and gain stages to reduce their inherent problems. If you want a unity gain buffer make one with an appropriate low noise unity gain op-amp. Bob

Excessive?

On the other hand, audio folks love open-loop cathode followers, which are far more nonlinear than any reasonably biased emitter follower.

John

It's daft ! For starters you'l get a 'DC thump' every time the follower stage is switched on or off.

JFETs make very very acceptable audio switches when used correctly.

Graham

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op-amps,

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Don't you mean Ear-Queers? They like non-linear circuits. Some of them even prefer single ended triode power stages sans feedback...go figure. Some distorions can be perceived as quite musical. After all if you are just adding to or rearranging a few harmonics in an already harmonically rich signal, who's to say it doesn't sound better?

Thanks again for all the replies.

stage is

OK this answers one of my other possible problems - I wasn't sure if the switching on/off was going to cause a bigger noise than the relays popping. I had planned on ramping up the voltage to the follower, but I don't know if that would help or not.

I'm intrigued. Any suggestions on how to use one for this application?

Thanks

Dead easy.

Use a nice low Ron jfet like a J108 and pull it off with a negative voltage on the gate.

Use an open-collecter arrangement to drive the gate with say 10M from gate to source. The signal path is from drain to source ( bidirectional ). Use a 10k 'pull down' R on both in and out to avoid any DC levels getting into the switch.

If finding a negative voltage is a problem use a p-channel device like a J174 and pull the gate positive.

The n-channel parts have lower Ron as do the lower numbered devices in each family. The tradeoff is a higher Vgs to turn them off.

Graham

Audiophools(tm)

Graham

You could also use a constant current load too.

Graham

Even better. At 10 mA emitter current, output impedance will be under

John

With Re as low as that you'll most likely be seeing the reflected ( terminology ? ) impedance of the previous stage in fact.

Graham

To drive the output load the stage needs a varying current, which in return requires a slightly different Vbe. The smaller the load, the more the variation in Vbe, which has an exponential characteristic. The inherent feedback helps linearizing, but for lower R_load and higher capacitive loading distortion shows up immediately.

That would be a cure, but then any load impedance will annulate the effort.

The output impedance is (1/gm + Rg/Hfe)||Re. With 50k input impedance we will hardly come below 100R. So even if the simple follower has a very high input and low output impedance, its properties cannot be used simultaneously. Those disadvantages can be overcome with some more sophisticated circuitry like the White-follower or this differential input stage: +---+------o | | +Vb / \\ | ( 1m) | \\_/ | | | | |/ +-| | |>

| | .---)---+ out | | +------o |/ \\| | o---| |-+ |>

Hi Ee

I actually discovered the J105 and was thinking of trying to use it somehow already.

I haven't really worked with JFETs much before, and am only now becoming familiar with the open collector thing (which would be open drain in this case?). Could we move this discussion to ABSE and I can post a schematic of what I think it should be set up like? I'm also unsure of the biasing required.

Thanks

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