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Re: To match or not to match... BS170s in your RXTX

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  • warrenallgyer
    Hi Taka Here is what I see in this circuit: There is a long voltage divider chain from the 12V rail consisting of Q3, R37(22.1K),Q5, and R41(33.2). As Q5 is
    Message 1 of 19 , Nov 30, 2012
      Hi Taka

      Here is what I see in this circuit:

      There is a long voltage divider chain from the 12V rail consisting of Q3, R37(22.1K),Q5, and R41(33.2). As Q5 is driven more and more into conduction by heat from the Q5/Q7/Q8 combo two things happen:

      1) As discussed, current through Q5 drives Q4 and effectively reduces the resistance of R40. This lowers the bias level on the finals.

      2) The increased current through Q5 also increases the voltage drop across R37 which reduces the collector voltage on Q6, the 2n2222 driver. This reduces the drive level to the finals, reducing the power output.

      So variances in Q5 produce big variances in the Q6 drive level as well as the operating point of the finals. This is why the Q5 circuitry is so effective at stabilizing the finals and why a choice here affects the power performance of the radio.

      This is my analysis. Do you agree?

      Warren Allgyer - W8TOD

      --- In softrock40@yahoogroups.com, "Taka Sugi" <taka_sugi1206@...> wrote:
      >
      >
      >
      >
      >
      >
      > Hi Warren
      >
      > I don't understand why Q5 dramatically affects output power.
      > Because, Q5/Q4 circuit works as constant voltage circuit to feed bias voltage to PA: Q7,Q8.
      > Q5 and Q7/Q8 are thermally connected, if Q5 is different maker/lot from Q7/Q8, then output power-temperature dependence will be changed.
      >
    • KF4BQ
      ... Hi Warren, Some bias point operation and other issues discussed at: http://groups.yahoo.com/group/softrock40/message/55570 73, Mike Collins KF4BQ
      Message 2 of 19 , Nov 30, 2012
        --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@...> wrote:
        >
        > So variances in Q5 produce big variances in the Q6 drive level as well as the operating point of the finals. This is why the Q5 circuitry is so effective at stabilizing the finals and why a choice here affects the power performance of the radio.
        >
        > This is my analysis. Do you agree?
        >

        Hi Warren,

        Some bias point operation and other issues discussed at:
        http://groups.yahoo.com/group/softrock40/message/55570

        73, Mike Collins KF4BQ
      • Taka Sugi
        Hi Warren Yes, almost I agree. ... 1. Ib(Q4)= Id(Q5) - Vbe(Q4)/R41 Here Vbe(Q4) is about 0.6V 2. Ic(Q4)=hfe(Q4) x Ib(Q4) 3. Vbias= V1 - (R39 xIc(Q4) + Vd(D3))
        Message 3 of 19 , Dec 2, 2012
          Hi Warren

          Yes, almost I agree.

          > 1) As discussed, current through Q5 drives Q4 and effectively
          >reduces the resistance of R40. This lowers the bias level on
          >the finals.

          1. Ib(Q4)= Id(Q5) - Vbe(Q4)/R41
          Here Vbe(Q4) is about 0.6V
          2. Ic(Q4)=hfe(Q4) x Ib(Q4)
          3. Vbias= V1 - (R39 xIc(Q4) + Vd(D3))
          Here V1= S12v - (R37 xIbias-circuit + Ic(Q6))
          If Id(Q5) is increased, then Ic(Q4) is increased, as a result
          Vbias is decreased( goes down).

          If Id(Q7,Q8) goes up by temperature, then Id(Q5, thermally connected
          to Q7,Q8) also goes up and Vbias goes down, then Id(Q7,Q8) goes down.
          This is a kind of "feedback".
          I am not sure about the function of R40(22.1k-ohm),but I think this
          is to avoide "local oscillation"?

          > 2) The increased current through Q5 also increases the voltage
          > drop across R37 which reduces the collector voltage on Q6, the
          >2n2222 driver. This reduces the drive level to the finals, reducing
          >the power output.
          > So variances in Q5 produce big variances in the Q6 drive level as
          >well as the operating point of the finals. This is why the Q5
          >circuitry is so effective at stabilizing the finals and why a
          >choice here affects the power performance of the radio.
          >
          > This is my analysis. Do you agree?

          I think there is little influence to Ic of driver Tr:Q6(2N2222),
          unless Q5 varies drastically, because R37 is small ( 22.1 ohm).

          JA1TLH/Taka
        • Alan
          ... Subject: [softrock40] Re: To match or not to match... BS170s in your RXTX Taka, Did you read this? http://groups.yahoo.com/group/softrock40/message/55570
          Message 4 of 19 , Dec 3, 2012
            ----- Original Message -----
            Subject: [softrock40] Re: To match or not to match... BS170s in your RXTX


            Taka,

            Did you read this? http://groups.yahoo.com/group/softrock40/message/55570

            Mike points out that Q4 maintains a constant current through R41 and therefore through Q5.
            The current is constant in Q5, what varies is the voltage across it.
            This is passed to Q7/8 through D3.
            Q4 just maintains the constant current.

            Mike, are you there?

            73 Alan G4ZFQ



            > Yes, almost I agree.
            >
            >> 1) As discussed, current through Q5 drives Q4 and effectively
            >>reduces the resistance of R40. This lowers the bias level on
            >>the finals.
            >
            > 1. Ib(Q4)= Id(Q5) - Vbe(Q4)/R41
            > Here Vbe(Q4) is about 0.6V
            > 2. Ic(Q4)=hfe(Q4) x Ib(Q4)
            > 3. Vbias= V1 - (R39 xIc(Q4) + Vd(D3))
            > Here V1= S12v - (R37 xIbias-circuit + Ic(Q6))
            > If Id(Q5) is increased, then Ic(Q4) is increased, as a result
            > Vbias is decreased( goes down).
            >
            > If Id(Q7,Q8) goes up by temperature, then Id(Q5, thermally connected
            > to Q7,Q8) also goes up and Vbias goes down, then Id(Q7,Q8) goes down.
            > This is a kind of "feedback".
            > I am not sure about the function of R40(22.1k-ohm),but I think this
            > is to avoide "local oscillation"?
            >
            >> 2) The increased current through Q5 also increases the voltage
            >> drop across R37 which reduces the collector voltage on Q6, the
            >>2n2222 driver. This reduces the drive level to the finals, reducing
            >>the power output.
            >> So variances in Q5 produce big variances in the Q6 drive level as
            >>well as the operating point of the finals. This is why the Q5
            >>circuitry is so effective at stabilizing the finals and why a
            >>choice here affects the power performance of the radio.
            >>
            >> This is my analysis. Do you agree?
            >
            > I think there is little influence to Ic of driver Tr:Q6(2N2222),
            > unless Q5 varies drastically, because R37 is small ( 22.1 ohm).
          • Taka Sugi
            Hi Alan ... Yes, I knew. I analyed before and also simulated@by@LTspice. If you have an interest pse refer to...
            Message 5 of 19 , Dec 4, 2012
              Hi Alan
              > Did you read this? http://groups.yahoo.com/group/softrock40/message/55570

              Yes, I knew.
              I analyed before and also simulated@by@LTspice.
              If you have an interest pse refer to...

              http://dl.dropbox.com/u/85347415/HAM/temp/PA_Bias%20Circuit.doc

              JA1TLH/Taka
            • Alan
              ... Subject: [softrock40] Re: To match or not to match... BS170s in your RXTX Sorry Taka, I am too dumb to understand. But finally I see. I need simple
              Message 6 of 19 , Dec 4, 2012
                ----- Original Message -----
                Subject: [softrock40] Re: To match or not to match... BS170s in your RXTX


                Sorry Taka,

                I am too dumb to understand.

                But finally I see. I need simple explanations without mathematics.
                Q4 is configured to pass a constant current through R41. It does this by controlling Q5.
                So Q5 source/gate voltage is set by Q4 so it will pass that constant current keeping Q4 base at ~0V7.
                Q5 gate voltage will vary according to the temperature of Q5 (in thermal contact with Q7/8) and is sent to Q7/8 with D3 dropping
                ~0V7 to compensate for Q4 base voltage.

                Simple, but not as simple as me.

                73 Alan G4ZFQ
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