```Sam Goldwasser (sam@stdavids.picker.com) wrote:
: In article <4qtau3\$9l@jagalchi.cc.pusan.ac.kr> ysaum@hyowon.cc.pusan.ac.kr (Youngsub Aum) writes:

: >   Thank you 4 read this
: >   I want 2 have Amplifier which have 100 gain.
: >   Just use NPN transistor.

: >   Theoritical design is not a Perfact Answer.
: >   What make it deficult?
: >   Some help...... PLEASE

Which gain -- Av, Ai, or Ap??  I assume we're talking about Av.

: How close to 100 do you want it?  The nice thing about op amps is that with
: very high open loop gain, controlling the closed loop behavior is for many
: applications very accurate and the theory works.

: With a single NPN of high enough Hfe, you can get 100 gain easily - you will
: need to use an emitter resistor as feedback to stabilize the collector
: current vs. input voltage.  The gain will be roughly the ratio of the collector
: to emitter resistances.  However, since Hfe is not infinite and can vary
: quite a lot from transistor to transistor - or with temperature, etc. - this
: will not be nearly as constant as an op amp.

:                       o
:                       |
:                       /
:                       \
:                       / Rc
:                       \
:                       |
:                       +-----||-----o Output
:                       |
:                     |/ C
:    Input o----------|    NPN
:                     |\ E
:                       |
:                       /
:                       \ Re
:                       /
:                       \
:                      _|_
:                       -

: (Biasing not shown).

: Vout (small signal) ~= Vin * Rc/Re.  The Re results in the transistor
: operating as an emitter follower which means that Vin appears on Re.
: Since nearly the same current (actually, reduced by Hfe/(Hfe+1)) flows
: through Rc, the voltage across Rc will be Rc/Re times the voltage on Re.

The voltage gain is high only as long as the Rl on the output is >> Rc.

: With multiple transistors, however, you can design a very stable high gain
: amplifier.

The usual preamp stage has two transistors in a shunt series connection.
They can have nice flat gain with wide bandwidth.  The negative feedback
works wonders.  See H&H's Art of Electronics for more info, as he states
below.

: See any introductory book on transistor circuit design or Horowitz and Hill..

: --- sam

--
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```

Date: 6 Jul 1996 17:18:22 GMT

Original Subject: Re: Q: 100gain AMP without OP-AMP ??

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