Re: Collector to Base Resistor - Why?

Lumpy wrote:
Help me to understand the purpose of this
resistance, please.

Typical single Q NPN amplifier (guitar boost).

o--/\/\/--o +v
|
R? |
o--\/\/\--o--||-----o OUT
| /
| /C
| |/
IN o----||--o-----|B
|\
\E
\o--/\/\/--o
|
|
# GROUND

What is the R? doing in this circuit?

It provides a bias current to get the transistor conducting a little, so that it can respond to the AC signal arriving through the base capacitor. Since it delivers a current roughly proportional to the collector voltage, the bias point is somewhat stabilized for different transistor gains (if a high gain transistor over reacts to the bias current by turning on too much, the low output collector voltage lowers the bias, reducing the effect of higher current gain).

Unfortunately for high impedance guitar source, the negative feedback does not only apply to DC bias current, but to the signal frequencies as well. The net effect is that the feedback tries to hold the input node at a more fixed voltage, lowering the input impedance of the amplifier. Guitar signals work best into high (relative to the guitar pickup) impedance amplifiers.

Taking it further, if I want to install back to
back clipping diodes at that C to B point, how
will that R? resistance interact?

The feedback resistor will limit the gain to some maximum value when neither diode is conducting. You also need to think about the DC voltage across this resistor, and what that will do to the diodes. For instance, you may need to add a capacitor in series with the diode pair, so that only AC signals are altered by those diodes, not the DC bias point.
.

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