Single transistor voltage amplifier

Discussion in 'The Projects Forum' started by darryl_co, Jan 30, 2011.

  1. darryl_co

    Thread Starter New Member

    Jan 30, 2011
    I am using two 555 to create a sweep oscillator.The range of voltage on control voltage pin of IC2 swings from 4.6 to 8.6 volts.How do I increase the range using additional transistor from 1 to 11 volts?If not then as much as lower than 4.6 and higher than 8.6 volts.I want an aotomatic operation so I cannot use a pot.
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    May not be possible to do it with an additional transistor.

    T1 base tracks the 4.7uF cap charge and discharge cycle which is from 1/3 Vcc to 2/3 Vcc or 4V to 8V. T1's emitter follows the base with approx 0.6V offset (due to Vbe forward value). That's why you get from 4.6 to 8.6V.

    Even if you use an op-amp to do the required scaled voltage transfer, the output swing limitations of a conventional op-amp (albeit a single supply type) would not get you to the required 11V upper limit with a 12V Vcc.

    I guess you could use a higher value for the Vcc to a control opamp and run the 555's at a voltage a few volts lower - say off a dropping regulator from the main Vcc. You could run with a Vcc of 15V with a 12 regulator to run the timers.

    All depends on your design / component count constraints, etc.
  3. darryl_co

    Thread Starter New Member

    Jan 30, 2011
    Is there no simple way to increase the swing range lesser than 4 greater than 8
  4. wayneh


    Sep 9, 2010
    Use a comparator and a reference voltage of, say, 6. The output of the comparator will flip from ground to Vcc. Most comparators require a pull-up resistor on the output to provide the high voltage.

    I think you can build a comparator from transistors, but I'd highly recommend just buying one such as the LM339, which is a widely available quad comparator on one IC.
    darryl_co likes this.
  5. russ_hensel

    Distinguished Member

    Jan 11, 2009
    Use 2 op amps one inverting, one non inverting and take push pull output to double the normal range. Not useful if you need a ground referenced output.