Class AB RF amp

Discussion in 'The Projects Forum' started by Regected, Sep 19, 2009.

  1. Regected

    Thread Starter Active Member

    May 24, 2009
    I was given an outside assignment this week in my communications class to build an RF amplifier for use with an AM transmitter lab the rest of the class is building. I had already finished my transmitter and hooked a microphone to it to test, but we were only able to get a very weak signal at best. I was told to use what parts I have in my parts kit to build a quick and dirty amplifier. I have basic transistor knowledge, but have not designed too many circuits yet.

    I've done a few hours worth of reading between my text book and google searches, and have come up with a very rough idea of where to start. I'm just going to use this simple circuit that I've seen in a few locations including wikipedia (I know, I know; don't use wikipedia). It's very close to the design i had in mind with the addition of Q3.

    For Q1, Q2 and Q4 I'm going to use 2n3904 and will use 2N3906 for Q5 and Q3. D1 and D2 will be 1n4148. The input comes from a XR2206 IC with a 2vpp 1600kHz signal. A supply of 20V will be used.

    I understand the operation of the differential amplifier of Q1 and Q2, but have not calculated the necessary values for the biasing resistors. How to calculate R7 and R8 is eluding me. Also, how do you figure R6? I guess it would help if i fully understood the operation of Q3.

    Any help in learning how to design this would be much appreciated. I will update with more information as I crunch numbers.
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005

    The amp example you show looks like an opamp or audio amp.
    RF amplifiers do not normally have feedback like that, they are usually much simpler open loop circuits.

    I'd do a google search for 'qrp transmitter' - this will turn up a vast selection of simple. low power transmitters and amplifiers.

    If you look at a complete transmitter circuit with two or more transistors, the last transistor stage is an RF amp (PA - Power Amp).

    QRP is an Amateur (Ham) Radio mode using a maximum of 5W of RF power, and very often only milliwatts from a tiny, simple transmitter.

    The 160m Amateur Band is at around 1.8MHz, pretty close to your frequency - there are some designs for that which will give reasonable component values. Circuits for 7MHz etc would be generically OK but need frequency-dependent components recalculating.

    Examples: (Many circuits, look down the page)

    And a document on design principles:

    The designs will need adapting to suit the frequency you are working with.
    The extra inductor and capacitor filters after the power transistor are to remove any harmonics from the signal and ensure they comply with legal requirements.

    If you think these circuits are too simple to be realistic, as an example a guy called Bob Chapman (callsign W9JOP) has worked every state in America using the above 'Tuna can' two transistor tranmitter design.

    As a final example, here's one with a simple logic chip & no transistors:

    Note I've mostly ignored the 'Class AB' stuff - that is far more complex and usually only used at rather higher powers; a few examples:
    Last edited: Sep 20, 2009
    PackratKing likes this.
  3. PRS

    Well-Known Member

    Aug 24, 2008
    I know you weren't talking to me, rjenkins, but that is great advice and thanks for the links. I once built a receiver for CB radio without any filtering. I was listening to a conversation clear as a bell but when my washing machine started it put clicks into the sound and, by the conversation, I knew the people at the other end could hear it too! They thought someone was 'messing around' with them.
    Last edited: Sep 20, 2009
  4. Regected

    Thread Starter Active Member

    May 24, 2009
    Thanks for all the info. I have read over most of the information you linked, but unfortunately I'm being limited in parts to what is contained in my school supplied parts kit. Inductors are limited and mosfets are nonexistent. If I were building this for myself, I would build something like the grenade.

    I also forgot to mention the circuit must fit on a breadboard.
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    OK, you say the input signal is 2Vpp and the supply will be 20V.

    There is no point having a gain larger than 10, which is easily accomplished with a single transistor.

    I'd go for the most basic single transistor common-emitter stage; an NPN transistor, 100 Ohm emitter resistor, 470 Ohm collector resistor, then bias with 1K base to 0V and 8k2?? base to V+. This should be adjusted to give about 12V on the collector.
    Input coupling should be via a 0.1uf cap from the oscillator stage.

    That gives some gain (about x5) so you will have about 10V p-p on the collector.
    It now needs matching to 50 Ohms, to the simple thing to do is add an emitter follower.

    Use another NPN, connect the base directly to the collector of that first stage, connect the collector to V+ and the emitter to 0V via a 100 Ohm resistor.

    That will get warm - it will be taking about 100mA so dissipating 2W between the transistor and load resistor. Make sure the components are adequately rated.

    The antenna connection will be from the emitter via another 0.1uf cap.

    Make sure the supply is extremely well decoupled or the amp may oscillate. Also make sure the grounding & signal connections between the oscillator and the amp are short and the ground adequate thickness.

    I can't guarantee it will work, but I think it has a fair chance. At least with the frequency you are using, it's easy to see the results on a 'scope.

    Note the power output increases with the square of the voltage, so that x5 voltage gain is giving you 25x the wattage, assuming the matching is the same (and this may well also be improved).

    You should get somewhere between 100 - 200mW out into a 50 Ohm load.
  6. Audioguru

    New Member

    Dec 20, 2007
    The opamp that you are making has its gain determined by: (R8/R7) +1.
    Q3 provides most of the voltage gain and the negative feedback from R7 and R8 will probably cause the circuit to oscillate at a high frequency due to phase shifts. A small capacitor between the base and collector of Q3 will reduce high frequency gain and stop the oscillation.

    RF circuits usually do not work when built on a breadboard. Use a compact pcb design or stripboard instead.