CHAPTER 5Transistor Circuits: OBJECTIVES ppt

CHAPTER 5 Transistor Circuits OBJECTIVES Describe and Analyze: • Need for bias stability • Common Emitter Amplifier Biasing • RC-coupled Multistage Amplifiers • Direct-Coupled Stages • Troubleshooting Introduction The DC bias values for VCE and Ic are collectively called the “Q-Point”. Because a transistor’s beta varies 2 to 1 or more from device to device, biasing circuitry needs to be designed so that the Q-point is not a function of beta. Likewise, the gain of a transistor amplifier should not depend on beta. Gain should be set by the values of external components such as resistors. Beta Changes with Temperature Not only does it vary from device to device, beta is also strongly dependent on temperature. Voltage Divider Biasing • Choose Rb1 & Rb2 so that: Rb1 || Rb2 << Re for the worst-case value of beta • Vb is fixed by Rb1 and Rb2, and: Ve = Vb – 0.7V • Re >> r’e. Therefore Ic = Ie = Ve / Re Biasing Example For a circuit like the one on the previous slide, calculate Vb, Ve, Ie, Ic, Vc, and Vce given:  = 50 Vcc =12V, Rb1 = 100k, Rb2 = 20k, Rc = 4k, Re = 2k, Vb = [Rb2 / (Rb1 + Rb2)]  Vcc = 12V / 6 = 2 Volts Ve = Vb – 0.7 = 2 – 0.7 = 1.3 Ic = Ie = Ve / Re = 1.3V / 2k = 0.65 mA Vc = Vcc - Rc  Ic = 12V – 4k  1.3mA = 6.8V Vce = Vc – Ve = 6.8V – 1.3V = 5.5V r’e = 25mV / Ie = 25mV / 0.65mA = 38.5 Ohms Is Re >> r’e? Is 2000 >> 38.5 ? Yes! Input Impedance Zin will not depend on  if: Rb1 || Rb2 << Re Voltage Gain: Unbypassed Re Av = rc / Re where rc = Rc || RL Gain is stable but low Voltage Gain: Bypassed Re Av = rc / r’e where rc = Rc || RL. But r’e = 25mV / Ie Gain is high, but changes with the signal current Voltage Gain: Compromise A trade-off between high gain and gain stability . CHAPTER 5 Transistor Circuits OBJECTIVES Describe and Analyze: • Need for bias stability • Common Emitter