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Friday, September 29, 2023

"The Art of Electronics" #8 -- Why Not a Simple Emitter Follower as The AF Output Circuit?

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Back when Dean KK4DAS and I were trying to find a suitable AF amplifier circuit for our High School Direct Conversion receiver project, we were debating what to use as the final.  One option was the standard NPN-PNP push-push amplifier (like in Figure 2.53 above)  -- an advantage with this one was that it would not require an AF transformer.  But we decided that this circuit would add complexity to a project that we were hoping to keep very simple. 

Another option was a simple common emitter amplifier with a transformer in the collector circuit.  This worked, and was simpler.  We ordered the transformers.  

In the midst of all this, at the local radio club hams asked us why we just didn't put a single emitter follower at the output to handle the impedance transformation to an 8 ohm speaker (sort of as in Figure 2.52 above).  They were convinced this would work.  I was not so convinced and pointed out that we had never seen such a circuit in any of the ham radio literature.  If this could be done, why hadn't the likes of Doug DeMaw and others used this circuit in their many, many rigs?  

This discussion kind of ended there (we opted for the common emitter transformer circuit), but I have thought about it from time to time.  A couple of weeks ago, when I got the second edition of The Art of Electronics, I found the above discussion of the use of this kind of emitter follower circuit.  You can see why this circuit has not been used.  Just to be sure, I built one in LTSpice.  Sure enough, it takes way too much current.   

Thank you, Horowitz and Hill! 


  1. It's not the same, but I've always been fascinated by the Zen amplifier design by Nelson Pass.


    I wonder if a Class A MOSFET design was used due to owning stock in Alcoa (lots of aluminum heat sinking required) :)

    1. see figure 5 in that link for the Zen schematic

  2. Nice topic, these speaker amps. Let's say we want about 200-400mW into 8 ohms with low current drain. And simple as possible (no Class D, yet!)
    1) You can use a little pre-bias on fig 2.53 with two diodes, and a little more circuitry , you solve cross-over distortion, with low current.
    A Class AB Speaker Amp.
    2) Some just drive fig 2.53 with an op-amp, but not a 741. Something with >10x slew rate of a 741, like a NE5532. Gets you through the cross-over region fast. No pre-biasing required.
    3) And then there's the one in the old Markus Electronic Circuits Manual (1971, pg 12)
    "DlSTORTIONLESS CLASS A". One transistor self-generates its own bias. Uses a transformer to 16 ohms and a power Germanium PNP! From Electronic Design,March 1964.
    Markus was the "Smorgasbord" of Circuit Manuals, but not nearly as detailed as Art of Electronics. The latter is used in undergrad physics classes.

    1. Thanks for mentioning the Markus book, interesting !


  3. Yep, thought that one would spark some interest!
    I generally go with #1 the Class AB Speaker Amp.
    Simple, junk box parts, one and done.
    There are articles with #2 using a fast op amp. And #3 from Markus is worthy of transient simulation. Better to go to the bench and see what it does. Who has a spice model for a 2N178, anyway?
    Markus's was the second most worn out reference book in our college library. Next to the BSTJ!


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