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VENUS INTERNATIONL COLLAGE OF TECHNOLOGY GANDHINAGAR Prepared by Satapara Nirav 130810109083 Differential Amplifier Model: Basic Represented by: A = open-circuit voltage gain vid = (v+-v-) = differential input signal voltage Rid = amplifier input resistance Ro = amplifier output resistance The signal developed at the amplifier output is in phase with the voltage applied at the + input (non-inverting) terminal and 180° out of phase with that applied at the - input (inverting) terminal. The Inverting Amplifier Configuration • The positive input is grounded. • A “feedback network” composed of resistors R1 and R2 is connected between the inverting input, signal source and amplifier output node, respectively. Voltage Gain The negative voltage gain implies that there is a 1800 phase shift between both dc and sinusoidal input and output signals. The gain magnitude can be greater than 1 if R2 > R1 The gain magnitude can be less than 1 if R1 > R2 The inverting input of the op amp is at ground potential (although it is not connected directly to ground) and is said to be at virtual ground. vs isR i R vo 0 1 2 2 vs isR i R vo 0 1 2 2 R vo vs Av 2 is vs R R 1 1 Input and Output Resistances Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs = 0 vx i R i R 2 2 11 But i1=i2 vx i ( R R ) 1 2 1 vs R R since v 0 in i 1 s Since v- = 0, i1=0. Therefore vx = 0 irrespective of the value of ix . Rout 0 Inverting Amplifier Input and Output Resistances Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs = 0 vx i R i R 2 2 11 But i1=i2 vx i ( R R ) 1 2 1 vs R R since v 0 in i 1 s Since v- = 0, i1=0. Therefore vx = 0 irrespective of the value of ix . Rout 0 Inverting Amplifier Input and Output Resistances Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs = 0 vx i R i R 2 2 11 But i1=i2 vx i ( R R ) 1 2 1 vs R R since v 0 in i 1 s Since v- = 0, i1=0. Therefore vx = 0 irrespective of the value of ix . Rout 0 The Unity-gain Amplifier or “Buffer” This is a special case of the non-inverting amplifier, which is also called a voltage follower, with infinite R1 and zero R2. Hence Av = 1. It provides an excellent impedance-level transformation while maintaining the signal voltage level. The “ideal” buffer does not require any input current and can drive any desired load resistance without loss of signal voltage. Such a buffer is used in many sensor and data acquisition system applications. The Summing Amplifier Since the negative amplifier input is at virtual ground, v v 1 i i 2 i vo 1 R 2 R R 1 2 3 3 Since i-=0, i3= i1 + i2, R R vo 3 v 3 v R 1 R 2 1 2 Scale factors for the 2 inputs can be independently adjusted by the proper choice of R2 and R1. Any number of inputs can be connected to a summing junction through extra resistors. This circuit can be used as a simple digital-to-analog converter. This will be illustrated in more detail, later. Instrumentation Amplifier NOTE Combines 2 non-inverting amplifiers with the difference amplifier to provide higher gain and higher input resistance. R vo 4 (va v ) b R 3 va iR i(2R ) iR v 2 1 2 b v v i 1 2 2R 1 R R vo 4 1 2 (v v ) R R 1 2 3 1 Ideal input resistance is infinite because input current to both op amps is zero. The CMRR is determined only by Op Amp 3.