View Single Post
Old 26th Oct 2022, 3:28 pm   #9
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Improving Selectivity In Vintage Receivers

The first circuit in Peter's attachment doesn't work. Zout from an emitter follower is a lot lower than the emitter resistor value. The transistor in an emitter follower works as a feedback amplifier, turning on as much collector-emitter current as necessary to make the emitter voltage try to follow the base voltage. So the circuit tries to be a voltage source of low output impedance.

In a better analysis, the impedance of whatever is applied to the circuit (in parallel with the bias resistors) is reduced by a factor of Hfe. Then add 'little Re' which is 25 Ohms divided by the quiescent current in mA.

An emitter follower with 2.2k in series on the way to the filter would be a much better attempt at matching. It also makes obvious the losses implicit in simple matching.

That website has a very good collection of DIY resources for crystal filters. Jack Hardcastle was a nice chap and I remember having regular chats with him at the Rochdale QRP doos. Also worth looking for a WW article by a Mr Pochet, and Wes Hayward published some articles in the ARRL and in one of his RF design books.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now