This is an improvement over the previous base-bias circuit which had an increase from 1.02mA to 3.07mA. We see that as beta changes from 100 to 300, the emitter current increases from 0.989mA to 1.48mA. Recalculate the emitter current for a transistor with β=100 and β=300. Find the emitter current I Ewith the 470 K resistor. The closest standard value to the 460k collector feedback bias resistor is 470k. It should be approximately midway between V CC and ground. Solving for I B yields the IB CFB-bias equation.įind the required collector feedback bias resistor for an emitter current of 1 mA, a 4.7K collector load resistor, and a transistor with β=100. Solving for I E yields the IE CFB-bias equation. Write a KVL equation about the loop containing the battery, R C, R B, and the V BE drop. This, in turn, decreases the emitter current, correcting the original increase. If the emitter current were to increase, the voltage drop across R C increases, decreasing V C, decreasing I B fed back to the base. Variations in bias due to temperature and beta may be reduced by moving the V BB end of the base-bias resistor to the collector as in Figure below. Thermal run away is the result of high emitter current causing a temperature increase which causes an increase in emitter current, which further increases temperature. The base-biased emitter current is not temperature stable. However, low level signals will not be clipped.īase-bias by its self is not suitable for high emitter currents, as used in power amplifiers. The bias point will still drift by a considerable amount. However, for low level signals from micro-volts to a about a volt, the bias point can be centered for a β of square root of (100♳00)=173. This is not acceptable in a power amplifier if we expect the collector voltage to swing from near V CC to near ground. However, with a change in β from 100 to 300, the emitter current has tripled. The emitter current is little changed in using the standard value 910kΩ resistor. What is the emitter current with a 910kΩ resistor? What is the emitter current if we randomly get a β=300 transistor? Solving the IE base-bias equation for R B and substituting β, V BB, V BE, and I E yields 930kΩ. Assuming that we have a β=100 transistor, what value of base-bias resistor is required to yield an emitter current of 1mA? Silicon small signal transistors typically have a β in the range of 100-300. If β is large we can make the approximation that I C =I E. Note that we use V BB for the base supply, even though it is actually V CC.
Write a KVL (Krichhoff’s voltage law) equation about the loop containing the battery, R B, and the V BE diode drop on the transistor in Figure below. A similar circuit is shown in Figure below. Note the resistor from the base to the battery terminal. An example of an audio amplifier stage using base-biasing is “Crystal radio with one transistor. It is convenient to use the existing V CC supply instead of a new bias supply. The simplest biasing applies a base-bias resistor between the base and a base battery V BB.
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