I believe it is still an emitter follower. The voltage gain=1 and the current
gain is >1 (both idealized) which is basically the definition. Some people
call it a "voltage buffer" or "common collector". Basically the voltage at the
emitter relative to a reference point is the same as that at the base relative
to the same reference point, but it is able to sink significantly more current
than the Zener alone.
I agree that the 78xx and 79xx type devices are a marked improvement for
voltage regulator applications!
Al
AB2ZY
________________________________________
From: Amps <amps-bounces@contesting.com> on behalf of Manfred Mornhinweg
<manfred@ludens.cl>
Sent: Tuesday, February 3, 2015 2:11 PM
To: amps@contesting.com
Subject: Re: [Amps] Need Zener for 2KD-3
Al,
I think you mean a different circuit than the one I meant.
> Just FYI, this circuit is called an emitter follower. Back in the early
> 70's, this was the "regulated" power supply of choice when I worked in a lab
> - a 5 or 12 V Zener diode, depending on the application, a 2N3055 transistor
> and a suitable resistor.
That sounds like the typical 1960's style three terminal regulator: Unregulated
input to collector, and to a resistor, the resistor goes to base and the zener,
the other side of the zener to ground, and output taken from emitter.
What I meant, instead, is a two-terminal direct replacement for a high power
zener, that has the zener connected between collector and base, and a resistor
(much lower value than for the 3-terminal regulator) between base and emitter.
This circuit actually has better regulation than the original high power zener!
And yes, the simple three terminal regulator had rather poor regulation, because
varying input voltage caused strong variations of zener current, and thus
significant variations of zener voltage, and because at very low or zero load
the leakage through the transistor (specially with early power transistors!)
would make the output voltage soar almost to the level of the unregulated input!
I remember many consumer devices having those simple emitter follower
regulators. For may applications they were good enough - just as a string of
diodes can be good enough to bias an amp!
Nowadays we have the luxury of IC three terminal regulators, designed in CMOS,
having extremely low dropout voltage, microamperes of internal current
consumption, better than 1% voltage accuracy, and better than 0.1% stability!
Alas, they are available in SMD packages only, so most of us can't manage to
find glasses thick enough to allow us to see them!
Manfred
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