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.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 Fall Semester – 2012 Transistor as an Amplifier Circuit: Lecture No: 20 Nasim Zafar Amplifier Gain: Ø • • Ø Ø Amplifiers are 2port networks: input port output port A is called the amplifier gain If the gain is constant, we call this a linear amplifier Nasim Zafar Transistor Specifications: Ø Maximum collector current, IC Ø Maximum power dissipated, PD Ø PD = IC * VCE Nasim Zafar Transistor Specifications: Ø Minimum CE voltage for breakdown, V(BR)CEO Ø Carefully examine absolute max ratings Ø DC current gain – variable – β = hFE in specs Nasim Zafar Amplifier Gain in Decibels: Ø Amplifier gain is expressed in decibels (dB) – Ø Ø 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 logbased 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.) Nasim Zafar Derivation of Decibels (Contd.): Ø Ratio of power of interest (call it “p1”) to some other reference power (say, p2): p1 p2 Ø However, these values are generally quite huge and tend to be logarithmically related; thus, creation of “the Bel:” p1 Bel = log p2 Nasim Zafar Derivation of Decibels (Contd.): • • 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: decibel power p1 = 10 log p2 Nasim Zafar Exercise: dB for Voltage: First, let’s relate voltage to power: p = vi i = v/r p=v r Nasim Zafar Exercise: dB for Voltage: Upon substitution: �v r � �v � �v1 � 10 log � �= 10 log � �= 10 log � � �v r � �v � �v2 � 2 2 2 Which gives us the decibel expression for voltage: decibelvoltage �v1 � = 20 log � � �v2 � Nasim Zafar 10 Transistor AmplifierOperation: Ø Ø Ø Amplification of a relatively small ac voltage can be achieved by placing the ac signal source in the base circuit. We know that small changes in the base current circuit cause large changes in collector current circuit. The small ac voltage causes the base current to increase and decrease accordingly and with the small change in base current ,the collector current will mimic the input only with greater amplitude Transistor AmplifierOperation: Ø The region between cutoff and saturation is called the linear region Ø A transistor which operates in the linear region is called a linear amplifier Ø Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input Amplifier OperationNPN Transistor1: Ø In this circuit, VBB forward biases the emitterbase junction and dc current flows through the circuit at all times Ø The class of the amplifier is determined by VBB with respect to the input signal Ø Signal that adds to VBB causes transistor current to increase Ø Signal that subtracts from VBB causes transistor current to decrease Nasim Zafar 30 Amplifier OperationNPN Transistor2: Ø During the positive peak of the ac input signal, VBB is added to the input Ø Resistance in the transistor is reduced. Current in the circuit increases Ø Larger current means more voltage drop across RC (VRC = IRC) Ø Ø Larger voltage drop across RC leaves less voltage to be dropped across the transistor We take the output VCE – as input increases, VCE decreases Nasim Zafar 31 Amplifier OperationNPN Transistor3: Ø Ø As the input goes to the negative peak: – Transistor resistance increases – Less current flows – Less voltage is dropped across RC – More voltage can be dropped across CE The result is a phase reversal – Ø Feature of the common emitter amplifier The closer VBB is to VCC, the larger the transistor current Nasim Zafar 32 NPN Common Base Transistor Amplifier1: Ø Ø Signal that adds to VBB causes transistor current to increase Signal that subtracts from VBB causes transistor current to decrease Nasim Zafar 33 NPN Common Base Transistor Amplifier2: Ø At positive peak of input, VBB is adding to the input Ø Resistance in the transistor is reduced Ø Current in the circuit increases Larger current means more voltage drop across RC (VRC = IRC) Ø Ø Collector current increases Ø No phase reversal Nasim Zafar 34 NPN Common Collector Transistor Amplifier: Ø Ø Ø Ø Also called an Emitter Follower circuit – output on emitter is almost a replica of the input Input is across the CB junction – this is reversed biased and the impedance is high Output is across the BE junction – this is forward biased and the impedance is low Current gain is high but voltage gain is low Nasim Zafar 35 Hybrid Parameters: =β = Slope of curve Nasim Zafar 36 Hybrid Parameters: hie = VB/IB Ohm’s Law hie =input impedance hre = VB/VC Nasim Zafar 37 Hybrid Parameters: hfe = IC/IB Equivalent of b hoe = IC/VC Nasim Zafar 38 PNP Common Emitter Amplifier: Nasim Zafar 39 PNP Common Base Amplifier: Nasim Zafar 40 PNP Common Collector Amplifier: Nasim Zafar 41 Summary: Ø Most transistors amplifiers are designed to operate in the linear region. Ø The commonemitter amplifier has high voltage and current gain. Ø The commoncollector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance ...Transistor as an Amplifier Circuit: Lecture No: 20 Nasim Zafar Amplifier Gain: Ø • • Ø Ø Amplifiers are 2port networks: input port output port... If the gain is constant, we call this a linear amplifier Nasim Zafar Transistor Specifications: Ø Maximum collector current, IC Ø Maximum power dissipated, PD Ø PD = IC * VCE Nasim Zafar Transistor Specifications:... Which gives us the decibel expression for power: decibel power p1 = 10 log p2 Nasim Zafar Exercise: dB for Voltage: First, let’s relate voltage to power: p = vi i = v/r p=v r Nasim Zafar Exercise: dB for Voltage: Upon substitution: