[sdiy] transistor amplifier questions

[Fredrik Carlqvist]

I'm certainly no expert, but these are my rules of thumb:

* Usually you decide the average collector current first. Then you calculate
the base current by dividing by the transistor's hFE. Then you multiply this
by 10 to set the current in the base-bias net. The base current should be
neglible compared to the divider current.

* The input impedance will be roughly the two resistors in the base divider
paralleled plus the 'bootstrap' resistor.

* The practical limit will be about 1M input impedance. The lower the
collector current, the higher the impedance.

Hope this is of any use.


Fredrik C



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Nils Pipenbrinck
Sent: Friday, December 12, 2003 12:52
To: Synth-Diy
Subject: [sdiy] transistor amplifier questions


Hi Folks..

I know.. this question looks like a homework-assignment, but it's not.

I just decided, that it's the time for me to understand how transistor
amplifiers work.

So I spend a while reading through all the book I have. I think I got all
the basics (biasing and feedback stuff), but there some things where I don't
have any idea. To make it a bit more interesting I tried to design a little
discrete unity gain ac-preamp circuit with high input impendance. It's a
simple two stage design:

The first tranny is just a common collector amplifier with positive ac
feedback and just a gain of 1.2 (I think that is called bootstrapped). The
second tranny is a emitter follower.

http://konsum.farb-rausch.de/~torus/tranny.gif
(resistor values not shown.. I'm still playing around with them).


Now I have a couple of questions:

* Is there a rule of thumb how the collector-current relates to the
base-bias network current? Right now I make sure that the collector current
is not 10 times larger.

* How do I calculate the input impendance? The first basis bias circuit has
a resistance of about 60k (each divider resistor has 120k). The bootstrap
resistor has 80k. I need to know the input impendance to calculate the input
capacitor and the frequency response.


* What's the practical limit of input impendance I can get with such a
design?

I played around with spice, and I was able to raise the resistance of the
bias circuit to large values I've never seen in other designs. That way I
was able to use small caps for bootstrapping and decoupling while the
theoretical upper frequency limit was still larger than 500kHz.

My guess is, that the spice didn't take the base-collector capacitance into
account (which works as negative feedback and would give a upper frequency
limit for such a circuit). Is that right?

  Nils