Basic Electronic Operational Amplifiers and Their Characteristics

Basic Electronic Operational Amplifier

Operational Amplifier Operational Amplifier An Operational Amplifier (Op-Amp) is an integrated circuit that uses external voltage to amplify the input through a very high gain.

Operational Amplifier Operational Amplifier The actual count varies, but an Op-Amp contains several Transistors, Resistors, and a few Capacitors and Diodes. For simplicity, an Op-Amp is often depicted as this: Positive Power Supply Inverting Input - Output + Non- Inverting Input Negative Power Supply

741 Op 741 Op- -Amp Schematic Amp Schematic

Voltage Gain Voltage Gain The gain of the Op-Amp itself is calculated as: G = Vout/(V+ V-) The maximum output is the power supply voltage When used in a circuit, the gain of the circuit (as opposed to the op-amp component) is: Av = Vout/Vin

Op Op- -Amp Saturation Amp Saturation The maximum output value is the supply voltage, positive and negative. The gain (G) is the slope between saturation points. Vout Vs+ Ideal op-amp + AVin ~ Vin Vout Vin Zout=0 Vs-

Ideal Op Ideal Op- -Amp Characteristics Amp Characteristics Open-loop gain G is infinite Rin is infinite Zero input current Rout is zero

Ideal Op Ideal Op- -Amp Analysis Amp Analysis To analyze an op-amp feedback circuit: Assume no current flows into either input terminal Assume no current flows out of the output terminal Constrain: V+ = V-

Inverting Amplifier Inverting Amplifier Rf (1) Kirchhoff node equation at V+ yields, 0 = + V Ra Vo ~ Vin Kirchhoff node equation at V yields, _ in R (2) + V V R V V + = 0 o a f (3) Setting V+ = V yields V = R f o Notice: The closed-loop gain Vo/Vin is dependent upon the ratio of two resistors, and is independent of the open-loop gain. This is caused by the use of feedback output voltage to subtract from the input voltage. V R in a 9

Noninverting Amplifier Noninverting Amplifier (1) Kirchhoff node equation at V+ yields, V = + Vin + Vo iV Kirchhoff node equation at V yields, 0 + a R (2) Rf Ra R V V V = 0 o f (3) Setting V+ = V yields R R V V V V f =1 + + or 0 = o i i o R V R a f i a

Summing Amplifier Summing Amplifier (1) Kirchhoff node equation at V+ yields, 0 = + V Rf Ra Va Vb Vc Rb Rc Vo Kirchhoff node equation at V yields, (2) + V V R R R V V V V V V _ o + + + = 0 a b c R f a b c (3) Setting V+ = V yields = f o R V V c V V V = j j + + = a b c R f R R R R a a b c j 11

Op Op- -Amp Buffer Amp Buffer Vout = Vin Isolates loading effects B A Low input impedance High output impedance

Op Op- -Amp Differentiator Amp Differentiator R C 0to Vi t1 t2 Vo 0 + t2 t1 to 13

Op Amp Integrator Op Amp Integrator

Op Op- -Amp Differential Amplifier Amp Differential Amplifier If R1 = R2 and Rf = Rg:

Common Common- -Mode Rejection Ratio Mode Rejection Ratio (CMRR) (CMRR) Differential voltage input : + = V V Vd Noninverting Input + Output Inverting Input Common voltage input : 1 = V Vc ++ ( ) V 2 Common-mode rejection ratio: G G = = d d CMRR 20 log ( dB ) Output voltage : = 10 G G c c + V G V G V o d d c c Note: When Gd >> Gc or CMRR Vo = GdVd Gd : Differential gain Gc : Common mode gain 16

Ideal and Practical Op Ideal and Practical Op- -Amp Amp Ideal op-amp + AVin ~ Vin Vout Zout=0 Practical op-amp + Zout Zin Vout Vin ~ AVin