From: email@example.com (Tom Bruhns)
Subject: Re: what is cascode concept
Date: 10 Dec 2002 10:28:55 -0800
NNTP-Posting-Date: 10 Dec 2002 18:28:55 GMT
firstname.lastname@example.org (eric) wrote in message news:...
> new to this group, I am trying to find out what is cascode concept.
> anyone knows? please provide idea. thank you very much
There's quite a bit of internal capacitance from the collector (or
drain or plate) of a transistor (FET, tube) to the base (gate, grid).
That capacitance affects the circuit performance quite a lot, for a
grounded-emitter (source, cathode) stage, because typically the output
voltage is much larger than the input voltage. That large voltage
across that capacitance results in feedback. It's especially
significant at high frequencies...a relative thing, because the
cascode can be used to advantage even at audio frequencies. Also,
that capacitance (especially in a bipolar transistor) varies with
voltage in a non-linear way, which can cause distortion. By adding
the second grounded-base stage, you isolate the high output voltage
from the collector of the first stage. The first stage collector in a
cascode stays at nominally a fixed voltage. The high voltage is only
at the output of the second, grounded-base stage.
It can also be used to let you use a transistor with desirable input
characteristics in a circuit that needs too high an output voltage for
your input transistor to handle.
The input and output devices don't have to be the same type; you can
use a bipolar driving an FET, or even mix in a tube/valve if you want.
And you can have a "folded" cascode, where the input is an NPN
transistor, and the output is a PNP.
Another thing you can do to reduce the effects of capacitance even
further is to bootstrap the base of the second stage, so that it
follows the input voltage. That results in extremely low effective
input capacitance. You can see an example of that in the old
(ancient??) GE Transistor Manual.