That pull down makes the system stable during start up. By itself, an amplifier operating in class B mode is not very useful. Class B: Bias is such that half (180) of the waveform is reproduced. If you have a pull down on the base of the transistor, then the stray voltages will have to be very substantial to make the motor run. The transistor spent half its time in active mode and the other half in cutoff with the input voltage too low (or even of the wrong polarity) to forward-bias its base-emitter junction. If there's any electrical noise in the vicinity, enough current may flow through the base of the transistor to cause the motor to run. Until your code executes, the output is floating. 1.For emitter bias, the voltage across the emitter resistor is the same as the voltage between the emitter and the 2. A version of the common-emitter circuit: Transistor Curves. R2 pulls the base of Q1 down to zero volts so that the load is off when there's no input on R1.Ĭonsider a microcontroller driving a small motor through a transistor.Īt power on, the output pins of many modern microcontrollers can be undefined - they are high impedance (open) until your code says to set them to output mode at a particular level. I was reviewing/following a tutorial describing the pros/cons of various transistor base biasing configurations. active region can be used as an amplifier we can look at collector characteristic curves. Imagine for a moment a simple transistor circuit involving a single NPN transistor with its emitter grounded, its collector tied to the positive supply by a resistor, and a potentiometer. That can randomly turn your load on or off - or somewhere inbetween. If R1 is left open (remove the signal generator from your diagram,) then stray voltages and currents may appear on the base of Q1. BJT AMPLIFIERS (1) Amplifier Operation (2) Transistor AC Models (3) The Common-Emitter. Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its DC operating point (the no signal values of. If you have a high or a low on the input at R1, then the circuit works like you expect - it switches the load on when the input is high and off when it is low. Biasing circuit: The biasing circuit consists of action of 3 resistances, R 1, R 2 and R E. (4)Emitter-Feedback Bias (5) CollectorFeedback Bias 5 Page. R2 is there so that the circuit is in a defined state when there's no signal on R1.
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