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    Bidirectional HF amplifier question

    Here is the diagram of an HF preamplifier. The single direction version at the left consumes about 30mA (biased in class-A for less distortion.
    Two questions:

    1. Will the hard biased left version mean in general a higher dynamic range apart from low distortion?

    2. Will the bidirectional right version work? Note only one of the two transistors will be switched for each direction and the other will stay off (not feeding any DC to its collector). Even if only one section will be on, I am still worrying about interactions between the two stages.
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    Re: Bidirectional HF amplifier question

    This amplifier will probably oscillate.. Watch out..
    The better case is to use either 2 RF switches ( TX/RX) or use 2 amplifiers connected as shown but one of them should be truned off while th second one is powered on.
    And if the frequency is high, they will impact the matchings between each other..


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    Re: Bidirectional HF amplifier question

    1. The headroom of that amplifier is not better than a simple class-A amplifier, but not worse. Biggest advantages are the negative AC feedback which improves linearity, and the increasing temperature won't cause increased operating current.
    2. I agree, this is an overcomplicated astable multivibrator not bidirectional amplifier.
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    Re: Bidirectional HF amplifier question

    You might get away with it if you reverse the supply on the 'off' amplifier. For example switching from +9V for ON to -9V for off but the complexity outweighs the advantages.

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    Re: Bidirectional HF amplifier question

    -9V supply exceeds the 2N2222 Vbe breakdown voltage (about -7V) and puts the CB junction in forward operation, creating a low impedance path between in- and output. That's not what you want.


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    Re: Bidirectional HF amplifier question

    Thanks FvM - I didn't look at transistor specs and you are quite right about the problem it would cause. A lower than breakdown voltage might still work though by preventing self biasing in the 'off' direction.

    When I have used signal switching in similar situations I used diode switching to isolate and ground the unwanted signal path.

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    Re: Bidirectional HF amplifier question

    Quote Originally Posted by betwixt View Post
    Thanks FvM - I didn't look at transistor specs and you are quite right about the problem it would cause. A lower than breakdown voltage might still work though by preventing self biasing in the 'off' direction.

    When I have used signal switching in similar situations I used diode switching to isolate and ground the unwanted signal path.

    Brian.
    Note the voltages TX/RX. It is not clear, but I mean that either TX will be fed or RX, depended on the direction of amplification, but not both!
    on transmit, only the TX voltage will be fed and the RX voltage will be zero (not connected to anything).
    similarly, on receive, only the RX voltage will be fed and the TX voltage will be zero (not connected to anything).

    I am not connecting both the TX and RX to 9v simultaneously!
    I am sorry this was not clear.

    So will this work?
    I worry about interactions of the switched-off amplifier stage (100nF, 14k, 100nF) that might affect the switche-on stage
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    Re: Bidirectional HF amplifier question

    We understand what you are trying to do. There are two issues, the first is the signal itself biasing the UN-powered stage, particularly in TX mode. It wouldn't work properly but if it had a gain of 1 or more it would oscillate. The second issue is simply the parasitic capacitances and inductances in the loop around the powered part. The phase shifting through theUN-powered part could still form an undesired feedback path.

    This is why I suggested applying a reversed supply voltage although as FvM pointed out, it has to be lower than the transistor breakdown voltage. The reversed supply will help to prevent the self biasing problem and at the same time might 'varicap style' reduce some capacitance in the remaining feedback path.

    Commercially, the method used is to make two amplifiers but use diodes to steer the signal through the appropriate path. The idea is that signal passing through a forward biased diode sees a relatively low series resistance but in a reversed biased path it sees only the residual junction capacitance. By suitably wiring diodes, typically three in each path, you can present two low resistances in series with a small capacitance between them to ground -or- two small capacitances in series with a low resistance between them to ground. It makes a very effective and simple signal switch.

    Brian.
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