MULTISTAGE AMPLIFIER Week 1 Tuesday 13 th July

MULTISTAGE AMPLIFIER Ø Week 1 Ø Tuesday (13 th July 2010) Ø BKN 4 1

MULTISTAGE AMPLIFIER Ø Many applications cannot be handled with singletransistor amplifiers in order to meet the specification of a given amplification factor, input resistance and output resistance. Ø For example, the required voltage gain may exceed that which can be obtained in a single-transistor amplifier circuit. Some particular design required low output resistance. Ø As a solution – transistor amplifier circuits can be connected in series or cascaded amplifiers. Ø This can be done either to increase the overall smallsignal voltage gain or provide an overall voltage gain greater than 1 with a very low output resistance 2

MULTISTAGE AMPLIFIER Ø Two or more amplifiers can be connected in a cascaded arrangement with the output of one amplifier driving the input of the next. Ø Each amplifier in a cascaded arrangement is known as a stage. Ø The basic purpose is to increase the overall voltage gain. 3

MULTISTAGE AMPLIFIER Multistage Voltage Gain Ø The overall voltage gain of cascaded amplifiers is the product of individual voltage gains. Ø Gain expressed in decibels (d. B) is Ø In multistage system, the overall voltage gain in d. B is the sum of individual gain 4

MULTISTAGE AMPLIFIER 5

MULTISTAGE AMPLIFIER Ø Given in previous slide is Capacitively-Coupled Multistage Amplifier. Ø Both stage are identical CE amplifier. Ø Capacitively coupling prevents dc bias of one stage affecting the other but allows ac signal to pass. Voltage Gain of the First Stage Ø Given Av = RC/r’e where RC is ac collector resistance and r’e is ac emitter resistance. Ø For the first stage, 6

MULTISTAGE AMPLIFIER Ø IE = 1. 05 m. A, r’e = 23. 8 Ω and Rin(base 2) = 3. 57 k Ω Verify all this! Ø So, the ac collector resistance for the first stage will be, Rc 1 = 4. 7 k Ω||47 k Ω||10 k Ω||3. 57 k Ω = 1. 63 k Ω Ø Thus, the gain of the first stage is 7

MULTISTAGE AMPLIFIER Ø Voltage gain of the Second Stage Ø Overall Voltage Gain 8

MULTISTAGE AMPLIFIER Ø Direct-Couple Multistage Amplifier 9

MULTISTAGE AMPLIFIER Ø No coupling or bypass capacitor. Ø Better low-frequency response than capacitively coupled type. Ø Used to amplify from low frequencies to dc (0 Hz). Ø No loss of voltage gain because no capacitive reactances in the circuit. Ø Disadvantage, small changes in dc bias voltage are amplified by succeeding stages. 10

MULTISTAGE AMPLIFIER Ø Exercise : obtain the gain for the circuit given below 11