More on Dino’s simple common emitter amplifier…

August 8, 2011 | electronics | By: Mark VandeWettering

I hadn’t worked through the biasing calculations for collector feedback common emitter amplifiers before, so I thought I’d do that for the simple one transistor NPN preamp that Dino built as his weeklyhack. But even before I got to it, I was confused by something. Check out the schematic (cribbed from Dino’s post):

I was confused by R1. It’s only purpose seemed to be to provide a DC offset, which was immediately removed by the action of C1. So, since I had built the circuit in LTSpice, I tried varying its value from 1 ohm to 100MEG ohms, and indeed, it had no effect on the overall gain of the amplifier. So, I removed it entirely. And the circuit continued to work just fine.

So, what’s the deal? I suspect that R1 was part of a circuit that powered an electret microphone: if the Input on his schematic were an electret microphone, you’d need to provide some voltage into it to power the built in FET preamp (like you see in the diagrams on the Wikipedia page for electrets). If that power is not needed, then all that happens is the current is dissipated by the input as heat. If you cascade two of these in the naive way, you end up a resistor feeding between voltage between two DC blocking transistors, which doesn’t waste a lot of power (no current flows at DC), but you end up with an extra cap and resistor.

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Comments

Comment from Aram H?v?rneanu
Time 8/8/2011 at 1:32 pm

I think it’s to stop current flowing into the input, I suspect the input might have had low impedance and the 10K R1 caused most voltage to drop on R1 instead of the input.

Comment from Mark VandeWettering
Time 8/8/2011 at 1:44 pm

If you simply remove R1, then the only current flowing back into the input has to be feedback current through the base. The 100K resistor drops the current to just a few microamps.

Comment from Chris Johnson
Time 8/9/2011 at 6:05 am

R1 does look redundant for a low-impedance input. The circuit could also have been designed for a carbon microphone (resistance varying with signal).

This circuit differs from the usual definition of a ‘common emitter’ amplifier, in that there is only one biasing resistor (R2). I don’t quite follow how this biasing works in this circuit… The art of electronics has a good description of biasing of the usual type:
http://books.google.com/books?id=bkOMDgwFA28C&lpg=PP1&pg=PA76#v=onepage&q&f=false

The other thing absent from Dino’s circuit is an emitter resistor – TAOE discusses the consequences of this too:
http://books.google.com/books?id=bkOMDgwFA28C&lpg=PP1&pg=PA82#v=onepage&q&f=false

Comment from wrm
Time 8/10/2011 at 6:30 am

Yup, I’d say that’s for powering the mike.

Comment from Luis
Time 8/11/2011 at 6:36 am

If you cascade two of these circuits, although the middle resistor may look like will do nothing, with the blocking caps become a high pass RC filter.

The positive rail is a low impedance path to ground and in small signal analysis power and ground are equivalent.

Certainly in this case as you comment is superfluous, but not always. Sometimes although unusual there are other reasons to put high value (megaohm) input resistors ahead of sensitive low leakage caps or high impedance nodes, static build-up.

Comment from Mark
Time 8/11/2011 at 8:18 am

Thanks Luis! Great comments! Very helpful!

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