CHAPTER 6: Other Transistor Circuits pdf

CHAPTER 6 Other Transistor Circuits OBJECTIVES Describe and Analyze: • Common Collector Amplifiers • Common Base Amplifiers • Darlington Pairs • Current Sources • Differential Amplifiers • Troubleshooting Common Collector Amplifiers The common-collector amplifier, more commonly called an emitter follower, is used as a “buffer” Buffers Amps The ideal buffer amplifier has unity voltage gain (Av = 1), infinite input impedance (Zin = ), and zero output impedance (Zout = 0). The power gain would also be infinite (Ap = ) The “job” of a buffer amp is to prevent loading of a signal source. If a high-impedance signal source is connected to a low-impedance point in a circuit, most of the signal will be lost in the source’s internal resistance. The buffer goes in between the source and the rest of the circuit. The Emitter Follower Buffer As we shall see, the emitter follower has a voltage gain slightly less than one (Av  1), a high input impedance (Zin  Re), and low output impedance (Zout  Re || Rb / ). It has a reasonably high power gain. Emitter followers are used very often in linear circuits, even in linear ICs. They are simple, yet effective. Biasing the Emitter Follower • Emitter followers typically use resistor divider biasing, just like the common-emitter amplifier. • Usually, the collector is tied directly to Vcc, so the collector to emitter voltage is Vce = Vcc – Ve. If Vcc is too high, then the transistor can get hot since the power dissipated is PD = Vce  Ic. Remember that Ic is basically equal to Ie = Ve / Re = (Vb – 0.7) / Re. • Emitter followers sometimes use a collector resistor to lower the Vce drop. A Biasing Example The Specifications: Suppose we have a circuit like that of figure 6-1. Vcc is 12 V, and we want Ve to be 6 V  5%. Find Rb1 and Rb2 so that there is 2 mA of current through Rb2 (Ib2 = 2mA). The minimum beta is 70. The emitter resistor is 600 Ohms (Re = 600). Also, find the power dissipation in the transistor. Biasing Example (cont.) • Find Vb: Vb = Ve + 0.7V = 6.0V + 0.7V = 6.7V • Find Rb2: Rb2 = Vb / Ib2 = 6.7V / 2mA = 3.35k • Choose a standard resistor value: Let Rb2 = 3.3k Find Rb1: [Rb2 / (Rb1 + Rb2)]  Vcc = Vb So, Rb1 = Rb2  [(Vc – Vb) / Vb] Rb1 = 3.3k  [(12V – 6.7V) / 6.7V] = 2.61k • Choose a standard resistor value: Let Rb2 = 2.7k Biasing Example Now let’s check to see if we got it right: Vb = [Rb2 / (Rb1 + Rb2)]  Vcc Vb = [3.3k / (3.3k + 2.7k)]  12V Vb = (3.3 / 6.0)  12 = 6.6V Maximum base current is 10mA / 70 = 0.14mA The Thevenin’s equivalent of Rb1 & Rb2 is RTH = (Rb1  Rb2) / (Rb1 + Rb2) = 1.5k and 0.14mA  1.5k =0.2V, so Vb = 6.6 - .2 = 6.4V But 5% of 6.7V is 0.34V. The minimum Vb is 6.36V, so Vb = 6.4V seems OK. Biasing Example Let’s find the power dissipation of the transistor: PD = Vce  Ic = (12V – 6V)  0.30A = 1.8 Watts Most likely, this guy needs a heat-sink. [...]... collectors are tied together, the transistors can not saturate When used as a switch, Vce  Ic can generate a lot of heat when Ic is big • A transistor s beta is often lower for very low values of Ic So the beta of Q1 may be a lot less than the beta of Q2 • The Vbe of a Darlington pair is 2  0.7 = 1.4V • The equivalent fT of the pair is lower than the fT of either transistor Common Base Amplifiers... very low, on the order of 50 Ohms But in a high-frequency amplifier, it’s usually required that Zin and Zout be around 50 Ohms Comparison of Configurations All three configurations have their place in circuits the base is at signal ground Differential Amplifiers The “diff amp” is commonly used in linear ICs Differential vs Single-Ended • All the amplifiers we have seen so far share one characteristic:... ground are called “common mode” signals Differential Amps & CMR • Suppose there is +10 mV (with respect to ground) on one input of a differential amplifier and –10 mV (with respect to ground) on the other input Then the differential signal is 20 mV If the diff amp has a gain of 10, the output will be 10  20 mV = 200 mV • Now suppose that +100 mV (with respect to ground) is applied to both inputs . CHAPTER 6 Other Transistor Circuits OBJECTIVES Describe and Analyze: • Common Collector Amplifiers • Common. Since the collectors are tied together, the transistors can not saturate. When used as a switch, Vce  Ic can generate a lot of heat when Ic is big. • A transistor s beta is often lower for very. directly to Vcc, so the collector to emitter voltage is Vce = Vcc – Ve. If Vcc is too high, then the transistor can get hot since the power dissipated is PD = Vce  Ic. Remember that Ic is basically