BJT NPN Buffer Question

[Box_Stuffer]

I was wondering if a BJT buffer needs a resistor inline with the power to the collector, like a BJT amplifier circuit.

I have a bunch of MPSA13 Darlington transistors so I made a buffer like the top one in the graphic. I just have the 9v running straight in and it seems to work okay.

[PRR]

a resistor inline with the power to the collector

Why? That's not where the signal comes out.

A wacky way to look at it: audio swings both ways. In some ideal world we'd always have two devices to pull this way and that way. In some *logic* families we do. But most *small* audio is much simpler with a device to pull one way and a resistor to pull the other way.

[antonis]

I was wondering if a BJT buffer needs a resistor inline with the power to the collector, like a BJT amplifier circuit.

You might find that resistor ONLY in very high input impedance circuits design..
But you'll find it either in Complementary Feedback pair configuration (setting first BJT Collector current) or splited in two resistors and bootstrapped via next stage voltage follower (for Collector output resistance high apparent value..)

In a single transistor buffer, it serves nothing other than Collector voltage (and consequently V_CE) limitation..

[R.G.]

A resistor in series with the collector of an emitter follower would matter only in secondary ways.

The collector of a BJT / BJT darlington is high impedance, meaning that it allows a current through, and the current is specified by the actions of the base-emitter. The collector voltage doesn't affect the current in the collector much at all as long as there is enough voltage and current available from the power supply to let the collector have the current it "wants".

But the voltage on the collector affects the operation of the base-emitter a little bit; this amount varies from device to device. You can reduce this by using a resistor from the power supply to the collector and a capacitor from the collector to ground. This filters any power supply noise fed to the collector so the noise and ripple don't feed into the base-emitter action and "pollute" the signal at the emitter.

This is a small effect, so for simple circuits that don't demand the best performance, it can be left off. There are a few other effects that happen for high frequency signals or very high impedance applications. But in general, it's not needed.

[Box_Stuffer]

Would a difference in voltage to the buffer change the sound? I do not have a 12v or 16v power supply so I cannot try it. I primarily use a 9v One Spot to power my breadboard.

[antonis]

Would a difference in voltage to the buffer change the sound?

It depends on what you mean with "sound change"..

Power supply deals only with signal amplitude, hence with clean or distorted output..

FWIW, for a Darlington transistor, I'd make R1 270k..

[ElectricDruid]

Would a difference in voltage to the buffer change the sound? I do not have a 12v or 16v power supply so I cannot try it. I primarily use a 9v One Spot to power my breadboard.

Assuming the input level stays the same, it would change the relationship between the input signal level and the transistor's transfer function. So, yes, it might change the sound. But only in the same way that turning down the input would do!
If you boosted the input level by the same amount at the same time as you boosted the power supply (and hence headroom) then I'd say, no, it won't change the sound.

[antonis]

Tom, we're talking about current amplifier here..
(not voltage amp)

[ElectricDruid]

Tom, we're talking about current amplifier here..
(not voltage amp)

Oh, sorry! I was late to the party...