# High gain using a push-pull

Discussion in 'Electronic Design' started by Kevin Weddle, Mar 6, 2004.

1. ### Kevin WeddleGuest

I am trying to design a push-pull for high gain. How high of gain can
I expect? I just want a general idea using two transistors. Also, I
can see that I will have a change in beta and a change in collector
resistance.

2. ### Winfield HillGuest

Kevin Weddle wrote...
Before you attempt to design, learn some of the basics about
bipolar transistors and amplifiers. You could do worse than
starting with Chapter two of AoE. For example, you'll learn
that a well-designed amplifier's properties are independant of
beta (because beta isn't predictable). Instead its open-loop
gain is dependent on g_m (or equivalently 1/r_e) times the
is generally much smaller than the BJT's intrinsic collector
resistance, which can be therefore be ignored (assuming any
common transistor with high Early voltage). Moreover, the
load resistance is often configured to be independent of the
actual external load, to insure predictable gain. Finally,
the use of local or overall feedback can over-ride all else.

To simplify, a common-emitter transistor amplifier stage can
be considered to be a form of impedance transformer, which
provides voltage gain as a function of the impedance ratios.
The gain for a BJT stage is G = g_m * load R. As we point out
in Chapter two, gm and load resistance scale with current, and
if the load resistance is chosen so the collector sits at half
the supply voltage, as is conventional, the maximum stage gain
= 20*Vs. With a 200V supply, the maximum gain would be 4,000.

But this is all pretty silly, because real amplifier circuits
have various criteria other than maximum gain, and are rarely
limited to one or two transistors, given their cost of only a
few pennies. For example, adding a current-source transistor
to the amplifier can boost the single-stage gain towards 1E6.
Furthermore, single stage high-gain transistor amplifiers have
*extremely* high distortion, because g_m varies with current
(see AoE page 83), and real amplifier designers of course use
various basic design tricks to avoid this problem, as we show.

Thanks,
- Win

whill_at_picovolt-dot-com

3. ### John WoodgateGuest

I read in sci.electronics.design that Winfield Hill
Why do I suspect that you are wasting your time? (;_)

4. ### Bill SlomanGuest

You probably need to search on the "Early effect" - the voltage of the
collector of a bipolar transistor effects the base-emitter voltage and
the current gain. Putting a pair of transistors in cascode can help.