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In some class C amplifiers in order to decrease conduction angle, transistor is biased with a negative voltage.

class C amplifier

Is it possible to create this negative voltage(-VBB) with positive supply? How?

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Is it possible to create this negative voltage(-VBB) with positive supply? How?

The simplest method is to raise the emitter by a couple of volts whilst keeping the base bias resistor connected to 0V. This clearly reverse biases the base but has the disadvantage of reducing collector-emitter voltage headroom. In many cases this may not be a problem.

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  • \$\begingroup\$ Thank you for answering. Is there a simple circuit with transistor, diode and capacitor that can actually provide this negative voltage. I'm looking for a simple circuit but without any IC's. \$\endgroup\$ – SMA.D Mar 3 '16 at 21:13
  • \$\begingroup\$ Use a potential divider to "force" the emitter to be a couple of volts above 0V and put a capacitor in parallel with the resistor at the lower end of the pot divider. Don't use high value resistors for this because the emitter signal current has to pass thru the effective impedance of the pot divider without much signal being developed. Signal developed = reduction of gain = less amplification, but it's a compromise. \$\endgroup\$ – Andy aka Mar 4 '16 at 0:51
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There are many options. A simple one is a voltage inverter like this:

enter image description here

Here the negative voltage is unregulated. Note that the circuit around the 555 only generates a square wave. If a suitable clock is already available in a product then this could be used as a clock source as well. Depending on how much current is needed from the negative rail a buffer might be needed to charge/discharge C2 more quickly.

There are also special ICs for this which include a feedback to stabilize the negative voltage.

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  • \$\begingroup\$ Thank you. Your answer is quite helpful. However I shouldn't use any ICs for my lab project! \$\endgroup\$ – SMA.D Mar 3 '16 at 21:29

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