Why is my CE amp not working? - Page 1
Forum section: electronics / beginners

author: DatKnight23
author_rank: Newbie
timestamp: April 30, 2021, 10:26:59 pm
content: I've been working on a simple morse transceiver for about 7.023MHz, I started out with a Peirce Oscillator from "Experimental Methods in RF design" Page 127, then added a emitter follower to buffer the signal coming from the collector, then a CE amplifier I designed to have a gain of about 15 with an approximately 200Ohm load. I mostly optimised for input resistance to match it to the output of the buffer, but then designed for the gain after that.When I turn it on however all I see at the output is an inverted version of the same input with half the amplitude (blue trace input yellow trace output on oscilloscope). Disconnecting the CE from the buffer I see the expected bias and emitter voltages at 2.4 & 1.8 volts each, but upon reconnecting the emitter voltage rises to 4.7V.I'm not entirely sure what I'm doing wrong here, but any advice would be appreciated.

author: jmelson
author_rank: Super Contributor
timestamp: April 30, 2021, 10:38:52 pm
content: Well, you have a 1 K collector resistor and a 200 Ohm load. That's the first thing I see. The collector resistor needs to be much lower value to get the full output. You might also put an inductor in series with the collector resistor and take the series resistor way down.Jon

author: DatKnight23
author_rank: Newbie
timestamp: April 30, 2021, 10:50:58 pm
content: Why would reducing the value of the collector resistor give me the full output? The gain is supposed to be determined by the ratio of the load the amp sees vs the equivalent emitter resistor no?

author: bob91343
author_rank: Super Contributor
timestamp: April 30, 2021, 11:19:10 pm
content: You have 2 or 3 mA collector current in the output stage. That's not enough to get the output you want. If you are driving 200 Ohms with, say, one Watt, class A, you need more collector current. Figure the load current and the collector current has to be greater. Also it's best to have a collector voltage of about half the power supply. You might consider getting rid of the collector resistor and use an inductor instead so you don't waste power.You might also want to take steps to reduce harmonic generation. Otherwise you may find you are also transmitting on 20 and 15 meters.

author: mikerj
author_rank: Super Contributor
timestamp: May 01, 2021, 08:18:24 pm
content: Quote from: DatKnight23 on April 30, 2021, 10:50:58 pmYou common emitter amp has an output impedance determined by the collector load, 1k in this case.

author: CaptDon
author_rank: Super Contributor
timestamp: May 03, 2021, 02:34:25 pm
content: Since you are driving the load off of the collector side you better lower the emitter resistor a bunch!! Your circuit looks like a 5 to 1 divider. It looks like even with the 10nf RF bypass cap on the emitter resistor the emitter voltage will rise with collector current draw and the base/emitter circuit is going to want to act like a diode and keep trying to self-lower the bias to near non-conduction. Everything about that 'final' amplifier looks wrong.

author: DatKnight23
author_rank: Newbie
timestamp: May 05, 2021, 06:47:26 am
content: You're correct that the emitter voltage was above the base, but how is this possible? Shouldn't the emitter be a diode drop below the base normally? What's happening when the signal is applied to make it jump up?

author: bd139
author_rank: Super Contributor
timestamp: May 05, 2021, 09:08:17 am
content: Numerous problems here. Firstly there needs to be an impedance transformation after the follower to the amplifier and you need to consider power rather than voltage amplification here. That's a very important distinction.Best bet as you're working through EMRFD is read through the first three chapters entirely and very slowly and the walk through for building the basic transmitter as that shows the design rationale of power amplification rather than voltage amplification. Make sure you are happy with the small signal model, large signal model, impedance transformation, building class C amplifiers, filters and power measurement first! Very important! This is a big learning curve and a headache and a half but it's enlightening. The is /not/ the same as building voltage amplifiers which is what you're trying to do here.