a way to use high gain transistors for some lower gain transistor circuits

[Gus]

Here is a simple way to use higher gain transistors for lower gain transistors NOTE you can only do this with some circuits
I simmed the first stage of a Shin-ei FY-2 for DC operating points
Part of the overall stock gain is 2.2meg / what is before it (guitar and cable or?)
So to get the collector at about the same voltage with a 2n5089 you need a 8.2meg
this changes the interaction/overall gain
What can we do
Ohms law, what stock value in parallel with 8.2meg will be about 2.2meg ?
If we AC couple it and make the cap large enough in value in the audio range, the AC and DC gain setting value will be close to 2.2meg
I used common value resistors

edit I spelled disconnects wrong

[Thecomedian]

 Usually, transistors have ranges of V and I that give "the best gain". If you use 2v and 100mA, for instance, on Transistor X, it'll have a gain of 300, but if you give it only 1v and 50mA, it's gain could be down to 50. Basically, you could use a high gain transistor in a "low gain circuit" if the transistor is outside it's comfort zone for Volts and Amps.

[MaxPower]

Doesn't voltage divider bias pretty much make transistor gain irrelevant? Might be an easier alternative.....

[Gus]

look at the sim I posted and also look up "inverting opamp circuit" etc

The signal at the collector is inverted compared to the input
So PART of the gain setting in the stage is the feedback resistor(the collector to base resistor(s)) in the circuits posted.  An opamp has more openloop gain than a transistor that is why I used the word PART

NOTE As I posted before this only works with gain stages like this.  Some people seem to like this kind of circuit so say you have higher gain transistor and you want to build something like a FY-2 and not change the stock resistor values.

A simple example lets MAKE UP a guitar a really simple one
7K pickup (no inductance)and 250k volume control
as you turn down the volume you are adding series resistance to the input so part of the gain could be 2.2meg / 100k or a gain of about 22
if you take the left side gain stage without the 3.3meg and the cap,  the gain at the same volume setting is 8.2/100k or a gain of about 82 add the 3.3meg resistor and cap and the gain is about 23.5


One FF like schematic has a series rheostat at the input that works with the first transistor and the feedback/bias is the Q2 E to Q1 B  resistor.

Look at the EM drive and all the other circuits like that

[Gus]

A bump
I thought this would get more posts and views

[Bill Mountain]

First time I've seen it.

Sometimes this forum moves too quick for me.

Is this actually lowering the gain of the input stage?

How is this different then just setting the stage up for less gain?

[Gus]

search for terms like "transistor gain stage shunt feedback"

You have the open loop gain of the transistor and the feedback part of the overall gain
PART of the stage's gain is the collector to base resistor divided by what is before the base of the gain stage

So if you are using a something like a MPSA18 instead of a 2N2222 the C to B resistor will be a higher value for the same DC bias.  This sets the gain at a higher value.

What I am trying to show is you can use a DC coupled higher resistor value to bias the transistor AND then reduce the gain with a parallel AC coupled C to B resistor.

[samhay]

This is a neat trick. I'm guessing the thermal noise in the large feedback resistor means that this has the potential to be noisier than simply tweaking the collector resistor?

[Gus]

I select the collector resistor for the desired collector current at the desired collector voltage