I notice George's first question is still unanswered.
>two questions occurred to me while catching up on
>my e-mail reading:
>1: "gain is mu*rp" ??? how??? gain is "mu" isn't it?
>i.e. Vout/Vin = gain = mu;
>or... gain = gm*rp, but not a mixture of the two!
>gain is dimensionless: mu is dimensionless:
>gm is a (trans)conductance with the units of inverse
>ohms: rp is expressed in ohms: gm*rp = dimensionless
Let's define terms - we've got into enough trouble on this reflector by
having words mean different things to different people.
mu (amplification factor) is dVp/dVg (where d is a Greek delta,
meaning a very small change). Similarly, gm (mutual conductance) is
dIp/dVg and rp (plate resistance) is dVp/dIp..........Vp is plate
voltage, Ip is plate current, Vg is grid volts.
The signal voltage at the plate is Vin*gm* [ rp*rl/rp+rl] where *
means 'times'.
rl is load resistance and rp is plate resistance
(I do wish you could guarantee e-mail to support equations!)
So the gain is Vo/Vin = gm* [ rp*rl/rp+rl]
The model is a current generator of Vin*gm with two resistors, rl and rp
in parallel across it.
But gm*rp = mu so gain is mu*rl/(rp + rl)
Does that clear it up George?
73
Peter G3RZP
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