Ian, the circuit Ive mentioned is very similar to your 2 transistor version.
I'll send it along with the other stuff once I get this scanner working
properly.
Carl
KM1H
----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Thursday, October 11, 2007 3:17 AM
Subject: Re: [Amps] CB Amps or full spectrum???
> Peter Chadwick wrote:
>>>BTW, a 1N5408 on top of an output transistor makes a fair temperature
>>regulator. Not perfect but its cheap and simple as a retrofit of an
>>existing brick.< If the diode is used just as a shunt regulator, that's
>>a not very good bodge. If it's used as asensor in a regulator, that's
>>better, although the thermal coupling is nowhwere near as good as one
>>would like.
>
> I've never understood why people drape a diode over the top of a
> transistor package. Everybody does it... but why?
>
> The diode cannot possibly sense short-term temperature changes inside
> the RF transistor. Inside the transistor, there is often an air gap
> between the silicon die and the underside of the ceramic top cap. To
> reach the diode chip, the heat has to jump that gap, flow through the
> thickness of the top cap, then through some random quantity of thermal
> grease, and finally through the glass or plastic encapsulation of the
> diode. If there is a sudden, severe overheating problem (eg because the
> antenna fell down), the transistor will be long dead before the diode
> could possibly sense any change.
>
> (Oh, and another thing: the diode is making a large pickup loop, over
> the top of an RF power transistor, with a rectifier right there in the
> loop. That doesn't seem like the best route to a predictable bias
> circuit.)
>
> The best practical location for a temperature sensor is on the top side
> of the mounting flange, which is very closely bonded to the transistor
> die. Also, a small tab-mounted power transistor makes a faster
> temperature sensor than a wire-ended diode (use the base-emitter
> junction). A small tab-ended SMD power transistor would be faster still.
>
> A simple active bias circuit can give better performance than a passive
> diode, and also save on bleed current. There is a classic two-transistor
> bias circuit (with design notes) at:
> http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm
> That circuit gives a reasonable compromise between performance and
> simplicity, and is a reasonable retrofit to existing bipolar amplifiers.
>
> Even then, the bias circuit can only track the relatively slow
> temperature changes that you'd expect in normal operation. That's the
> best you can possibly expect. For sudden problems, you'd better have
> other forms of protection.
>
>
> --
>
> 73 from Ian GM3SEK
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