UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Buffer Amplifier for Digital Frequency Counter

Radio Wrangler · 19th Apr 2021, 3:18 pm

I prefer that circuit to the Howes one. The howes circuit has the input linked straight to the gate of a MOSFET without any protection.

In the circuit you built, the 2N2222 common emitter amplifier has the emitter decoupled in an attempt to screw the last bit of gain out of it, which is not good for bandwidth. A better gain stage would be a cascode pair which should push the roll-off point up quite a bit.

Also the Zout of the Jfet may be a bit high and a limiting factor. An emitter follower to help with the loading of that flat out gain stage may help.

David

19th Apr 2021, 3:18 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: Buffer Amplifier for Digital Frequency Counter I prefer that circuit to the Howes one. The howes circuit has the input linked straight to the gate of a MOSFET without any protection. In the circuit you built, the 2N2222 common emitter amplifier has the emitter decoupled in an attempt to screw the last bit of gain out of it, which is not good for bandwidth. A better gain stage would be a cascode pair which should push the roll-off point up quite a bit. Also the Zout of the Jfet may be a bit high and a limiting factor. An emitter follower to help with the loading of that flat out gain stage may help. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done