On Sat, 16 Apr 2005 09:47:25 +0000 (UTC), "Reg Edwards"
wrote:

Bipolar and MOSFET amplifiers are by comparison (with tubes) low
impedance
devices

==============================
Actually they are not.

Please expand upon your remarks.

==============================

For example, the internal resistance, Ra, of a triode tube is -

Ra = Mu / Gm

where Mu is the amplification factor and Gm is the mutual conductance.

Typical values for a triode are Mu = 10 and Gm = 5 milliamps per volt
which gives Ra = 2000 ohms.

Corresponding values for a silicon NPN bipolar transistor are Mu =
1000 and Gm = 40 milliamps per volt which gives an internal resistance
of 25,000 ohms.

Typical values for a beam tetrode are Mu = 200 and Gm = 5 mA/volt
which gives Ra = 40,000 ohms.

For a field effect transistor Mu = 1000, Gm = 10 mA/volt and an
internal resistance of 100,000 ohms.

Typically, transistors have higher internal resistances than tubes.
The volts and amps at which they work, the power ratings, are
coincidental and have nothing to do with it except that when comparing
one device with another they should have similar power ratings.

Take a look at their DC characteristic curves. Internal resistance is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant base
current).

OK?

No.

All of this is very interesting, but you are referring to the
*devices* that are used in the amplifier, while I said, "Bipolar and
MOSFET *amplifiers* are by comparison low impedance.