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Input/Output Voltage Class C RF Amp

  1. Dec 16, 2015 #1
    Hi all:

    I am a new member here with my first post. I am an adult learner trying to self-teach RF electronics. I have been working through David Rutledge's Electronics of Radio, while building the NorCal 40A transceiver.
    I am getting caught up on understanding the Class C RF power amplifier circuit.

    I don't understand how the output voltage across the collector of Q7 can be more than twice the DC supply voltage (12VDC). I assume that the reactive components--such as RFC1--are storing energy?

    Any help would be appreciated. I have included the schematic below.

  2. jcsd
  3. Dec 16, 2015 #2


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    hi there and welcome to PF
    I cannot answer this completely
    1) interesting circuit, its pretty standard except for the zener diode across the transistor ... never seen that before
    2) consider that the 43V rating of the zener is just the voltage rating of the zener for when it will go into conduction and NOT the voltage that is
    present across the transistor all the time and most likely none of the time
    3) on the collector of the transistor, there is going to be an RF voltage as well as the DC supply voltage ... am not good enough to explain the complexities there
    4) RFC1 is a choke, whose sole purpose is to isolate the DC rail from the RF voltage at the transistor collector ... We DO NOT want RF getting onto the DC rail, it will cause all sorts of bad things to happen

  4. Dec 17, 2015 #3


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    Some comments:
    • The purpose of D7 is to protect the power supply
    • When S1 breaks the connection to the power supply, it causes a voltage spike across RFC1. The Zener is there to protect Q7 when that happens.
    • If the antenna is disconnected, the load on Q7 changes drastically. The Zener may help protecting Q7 in that case.
  5. Dec 17, 2015 #4


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    A Class C amplifier is working somewhat like a switch. When a current flowing in an inductor is broken, a high voltage can be generated equal to -L dI/dt. This is where the voltage comes from, and you are correct that it comes from energy stored in the tank circuit. The Zener is used to limit the RF voltage across the transistor to a safe value. Dangerous voltage could arise if the load resistance is accidentally omitted or the load is severely mismatched.
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