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Lecture Electrical Engineering: Lecture 20 - Dr. Nasim Zafar

The main contents of the chapter consist of the following: Most transistors amplifiers are designed to operate in the linear region; the common-emitter amplifier has high voltage and current gain; the common-collector has a high current gain and voltage gain of 1. it has a high input impedance and low output impedance. | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Transistor as an Amplifier Circuit: . Lecture No: 20 Contents: Introduction. Amplifier Gain. Common Emitter Amplifier. Nasim Zafar 2 Amplifier Gain: Nasim Zafar 3 Amplifiers are 2-port networks: input port output port A is called the amplifier gain. If the gain is constant, we call this a linear amplifier. Transistor Specifications: Maximum collector current, IC . Maximum power dissipated, PD PD = IC * VCE Nasim Zafar 4 Transistor Specifications: Minimum C-E voltage for breakdown, V(BR)CEO Carefully examine absolute max ratings. DC current gain variable β = hFE in specs. Nasim Zafar 5 Amplifier Gain in Decibels: Nasim Zafar Amplifier gain is expressed in decibels (dB) Originally it was expressed as “Bels” (named after Alexander Graham Bell), but these proved to be of insufficient size so we multiply “Bels” by 10 “decibels.” Decibels are a log-based ratio and are therefore dimensionless. Purpose: We want to measure the ratio of some value relative to another (e.g. sound power in a stereo amplifier). Derivation of dB (Cont.) 6 6 Derivation of Decibels (Contd.): Nasim Zafar Ratio of power of interest (call it “p1”) to some other reference power (say, p2): However, these values are generally quite huge and tend to be logarithmically related; thus, creation of “the Bel:” 7 7 Derivation of Decibels (Contd.): Nasim Zafar However, the Bel is a bit too small, so let’s multiply it by 10 and call it a decibel (10 x Bel = 1 dB). Which gives us the decibel expression for power: 8 8 Exercise: dB for Voltage: Nasim Zafar 9 First, let’s relate voltage to power: Exercise: dB for Voltage: Nasim Zafar 10 Upon substitution: Which gives us the decibel expression for voltage: Some Physical Conclusions: Nasim Zafar If dB is positive, then v1 > v2, the signal is amplified. If dB is negative, then v1 < v2, the signal is attenuated. If dB is 0, then . | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Transistor as an Amplifier Circuit: . Lecture No: 20 Contents: Introduction. Amplifier Gain. Common Emitter Amplifier. Nasim Zafar 2 Amplifier Gain: Nasim Zafar 3 Amplifiers are 2-port networks: input port output port A is called the amplifier gain. If the gain is constant, we call this a linear amplifier. Transistor Specifications: Maximum collector current, IC . Maximum power dissipated, PD PD = IC * VCE Nasim Zafar 4 Transistor Specifications: Minimum C-E voltage for breakdown, V(BR)CEO Carefully examine absolute max ratings. DC current gain variable β = hFE in specs. Nasim Zafar 5 Amplifier Gain in Decibels: Nasim Zafar Amplifier gain is expressed in decibels (dB) Originally it was expressed as “Bels” (named after Alexander Graham Bell), but these proved to be of insufficient size so we multiply “Bels” by 10 “decibels.” Decibels are a log-based