From: kevin@clsi.COM (Kevin Cameron)
Subject: Re: Grounded Emitter Amplifier Query
Originator: kevin@dubhe
Reply-To: kevin@clsi.com
Organization: CAD Language Systems Inc.
Date: Wed, 30 Dec 92 20:11:43 GMT


In article <1992Dec29.214231.1135@inmet.camb.inmet.com>, bwhite@cobra.camb.inmet.com (Bill White) writes:
> Hello.  I am reading Horowitz and Hill's book on Electronics, but 
> I am having trouble understanding the grounded emitter circuit.  In
> particular, I am having trouble understanding why the grounded emitter
> is not thermally stable.  I believe it, I just don't understand why
> it is true.  

	-----+-------------+---------------------- V_{CC}
             |             |
             |             |
             \             \
             /             / R_{C}
             \ R_1         \
             /             /
             |             |
             |             |
             |             +--------------------- V_{OUT}
             |             | (collector)
             |        +---------+
             |        |         |
      V_{B}->+--------+   npn   | (I've never seen a good
             | (base) |         |  ascii transistor.)
             |        +---------+
             |             | (emitter)
	     \             |
             /             |
	     \ R_2         |
             /             |
             |             |
             |             |
             |             |
           -----         -----
	    ---           ---
             -             -

It is true because the current gain of the device is very high. V_C needs to be
set at ~ V_CC/2, if the transistor gain is 200 V_CC = 10V and I_C = 5mA, R_C is
5kOhm, then R_1 (assume R_2 is infinite for now) is ~9.4V/(5mA/200) = 376kOhm to
bias it. A 1% change in gain gives a 1% change in I_C giving a voltage change of
50mV, 10% gives 0.5V. Considering that transistors are not supplied with a 
gain guaranteed any better than 50% and that it varies considerably with
temperature you can see why this is not a practical circuit (you assumed hFE was
constant). Adding R_2 makes things worse since it makes the bias current vary
more as Vbe changes.

Grounded emitter circuits are used mostly for AC amplification or switching.
The circuit below has DC feedback courtesy of R_E to stabilize the biasing.
At frequencies above a level defined by the R_E/C_E time constant the emitter
is effectively grounded and the device operates at maximum current gain.

> 
>         -----+-------------+---------------------- V_{CC}
>              |             |
>              |             |
>              \             \
>              /             / R_{C}
>              \ R_1         \
>              /             /
>              |             |
>              |             |
>              |             +--------------------- V_{OUT}
>              |             | (collector)
>              |        +---------+
>              |        |         |
>       V_{B}->+--------+   npn   |
>              | (base) |         |
>              |        +---------+
>              |             | (emitter)
   	       \             +----
               /             |    |
 	       \ R_2         \    |
               /             /   ___
               |       R_E   \   ___ C_E
               |             /    |
               |             |    |
>            --+-------------+----+---
> 

Kev.

PS. The new "H & H" has an extended section on switch-mode power supplies.

-- 
Kevin Cameron          CLSI, Suite 100, 5457 Twin Knolls Rd., MD 21045, USA
kevin@clsi.com         Tel: (USA) (410) 992 5700 (- 5709) x224
uunet!clsi.com!kevin   Fax:       (410) 992 3536
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