EMRFD Message Archive 6749

Message Date From Subject
6749 2011-10-17 17:41:58 Ray Emitter Follower Output impedance
For the circuit shown in Fig 4.23 of EMRFD, assume the dashed line is replaced with a 2.2k resistor to drive Q2. What would be the output impedance at the emitter of Q2? From what I've been able to determine reading about emitter followers, the impedance would be very low.

All of the explanations I've found on the internet use a capacitor between Q1 and Q2. They then set the input impedance and bias of Q2 with series resistors connected between Vcc and Gnd. The input impedance has an effect on the output impedance and the emitter resistor and transistor gain also affect the input impedance.

I've built the circuit using a 16 MHz crystal. For the 3 resistors I've used 1K instead of 2.2K. My scope shows 1.2 volts Pk-Pk across the Q2 emitter resistor. I want to use a PI pad to reduce the voltage to 250 mV and also to transform the impedance to 3K. I was thinking of using a 3K series resistor between the emitter and the pad. That should drop the voltage to .6 volts Pk-Pk if used to feed a pad designed for 3K input and output. A 7.5 dB pad would then drop the voltage to 250 mV if my calculations are correct.

I also want to take the signal from Q2's emitter and feed another stage through a 1K resistor. The drive level for that stage is not very critical and I can adjust it with a terminating resistor. I'm thinking the output level at Q2's emitter is large enough that the pad and the 1K resistor will have minimal impact on each other.

If I am approaching this the wrong way, what would be a better design?

6752 2011-10-17 20:59:31 Wes Re: Emitter Follower Output impedance
Hi Ray, et al,

The problem with a lot of the analysis that we read in classic treatments is that the calculations relate to SMALL SIGNAL output impedance. This is the case for the usual treatment of an emitter follower. But we are not dealing here with small signals.

One case that illustrates this is the RF emitter follower back in EMRFD Fig 2.31. That circuit will have a really low output impedance for small signals, but you can't generate any power with it. You can't drive an impedance that is still orders of magnitude higher than the small signal output Z.

This is a very comm
6753 2011-10-18 05:13:24 Ray Re: Emitter Follower Output impedance

Thanks for the explanation and reference. Those few paragraphs give the explanati
6754 2011-10-18 10:46:06 Wes Re: Emitter Follower Output impedance
Hi again Ray,

I re-read what what I wrote. Sorry for the sloppy presentation. I should read the text before I click on the blasted "Send" button. My fingers get ahead of my brain, or something geezerly like that. Sorry.

Yea, be careful, for even a little bit of power can get you in trouble. I was doing a breadboard just last night to look at generating LO power for a level 7 diode ring mixer. All I needed was a lousy 5 mW, yet I was running into problems. I kicked the bias current in a common-emitter stage from 3 mA up to 5 mA and it made all the difference. A follower would have taken 20 mA or so to do this same job, and this was not much power.

As I used to tell my kids, "You've got to think!" Now I'm hearing it from them.

73, Wes