09/16/2010© 2010 NTUST Today Course overview and information

The ideal op-amp is one with optimum characteristics, which cannot be attained in the real world. Nevertheless, actual op-amp circuits can often approach this ideal. The ideal op amp has infinite voltage gain, infinite input impedance (open), and zero output impedance. V in Z in = ∞ A v V in V out Z out = 0 +  The Ideal Op-amp

Practical op-amps have limitations including power and voltage limits. A practical op-amp has high voltage gain, high input impedance, and low output impedance. V in Z in A v V in V out Z out There are two inputs, labeled inverting and non inverting because of the phase relation of the input and output signals. +  inverting input non inverting input The Practical Op-amp

Virtual Short Circuit

Differential Input

Common-Mode Input

Common-Mode Rejection Ratio (CMRR)

Many times, noise sources will induce an unwanted voltage in a signal line. When the noise is induced in common-mode, the differential amplifier tends to cancel it. (The diff-amp cannot reject any signal that is in differential mode.) The ability to reject common-mode signals is measured with a parameter called the common-mode rejection ratio (CMRR), which is defined as CMRR can be expressed in decibels as Common-Mode Rejection Ratio (CMRR)

Examples

From the defining equation for CMRR: Expressed in decibels, it is A certain diff-amp has a differential voltage gain of 500 and a common-mode gain of 0.1. What is the CMRR? dB Common-Mode Rejection Ratio (CMRR)

The differential signal is amplified by 100. Therefore, the signal output is V out = A v(d) x V in = 100 x 50 mV = A certain diff-amp has A d = 100 and a CMRR of 90 dB. Describe the output if the input is a 50 mV differential signal and a common mode noise of 1.0 V is present. The common-mode gain can be found by The noise is amplified by Therefore, V noise = A cm x V in = x 1.0 V = 3.2 mV 5.0 V Common-Mode Rejection Ratio (CMRR)

Some important op-amp parameters are: Input bias current: Differential input impedance: Common-mode input impedance: Input offset current: Average of input currents required to bias the first stage of the amplifier: Total resistance between the inverting and non-inverting inputs Total resistance between each input and ground. Absolute difference between the two bias currents: Op-amp Parameters

Output impedance: Common-mode input voltage range: Common-mode rejection ratio Slew rate: The resistance when viewed from the output terminal. Range of input voltages, which, when applied to both inputs, will not cause clipping or other distortion. Ratio of the differential gain to the common- mode gain. The differential gain for the op-amp by itself is the same as its open loop gain. The maximum rate of change of the output in response to a step input voltage. Op-amp Parameters

Open-Loop Voltage Gain

Why Use Negative Feedback

Negative feedback In 1921, Harold S. Black was working on the problem of linearizing and stabilizing amplifiers. While traveling to work on the ferry, he suddenly realized that if he returned some of the output back to the input in opposite phase, he had a means of canceling distortion. One of the most important ideas in electronics was sketched out on his newspaper that morning.  + Internal inversion makes V f 180 o out of phase with V in Negative feedback network VfVf V in V out Quiz

Op amp circuits with negative feedback Negative feedback is used in almost all linear op-amp circuits because it stabilizes the gain and reduces distortion. It can also increase the input resistance.  + Feedback network VfVf V in RfRf RiRi V out A basic configuration is a noninverting amplifier. The difference between V in and V f is very small due to feedback. Therefore, The closed-loop gain for the noninverting amplifier can be derived from this idea; it is controlled by the feedback resistors: Quiz

Inverting Amplifier

 + V in RfRf RiRi V out The inverting amplifier is a basic configuration in which the noninverting input is grounded (sometimes through a resistor to balance the bias inputs). Again, the difference between V in and V f is very small due to feedback; this implies that the inverting input is nearly at ground. This is referred to as a virtual ground. The virtual ground looks like ground to voltage, but not to current! The closed-loop gain for the inverting amplifier can be derived from this idea; again it is controlled by the feedback resistors: Virtual ground Op-amp with Negative Feedback

Examples

The gain is 25 The feedback fraction is  + V in RfRf RiRi V out 36 k  1.5 k  What are the input and output impedances and the gain of the noninverting amplifier? Assume the op amp has A ol = 100,000, Z in = 2 M , and Z out = 75  Examples

 + V in RfRf RiRi V out The gain is 36 k  1.5 k   24 What is the input impedance and the gain of the inverting amplifier? Examples

The voltage-follower is a special case of the noninverting amplifier in which A cl = 1. The input impedance is increased by negative feedback and the output impedance is decreased by negative feedback. This makes it an ideal circuit for interfacing a high-impedance source with a low impedance load.  + V in V out Voltage-follower

Differential amplifier Common-mode rejection ratio (CMRR) Open-loop voltage gain An amplifier that produces an output proportional to the difference of two inputs. The internal voltage gain of an op-amp without feedback A measure of a diff-amp's or op-amp's ability to reject signals that appear the same on both inputs; the ratio of differential voltage gain or open-loop gain (for op-amps) to common-mode gain. Selected Key Terms

Negative feedback Closed-loop voltage gain Noninverting amplifier Inverting amplifier An op-amp closed-loop configuration in which the input signal is applied to the noninverting input. The return of a portion of the output signal to the input in such a way that it is out of phase with the input signal. The overall voltage gain of an op-amp with negative feedback. An op-amp closed-loop configuration in which the input signal is applied to the inverting input. Selected Key Terms

1. When two identical in-phase signals are applied to the inputs of a differential amplifier, they are said to be a. feedback signals. b. noninverting signals. c. differential-mode signals. d. common-mode signals. Quiz

2. Assume a differential amplifier has an input signal applied to the base of Q 1 as shown. An inverted replica of this signal will appear at the a.emitter terminals. b.collector of Q 1 c.collector of Q 2 d.all of the above. R C1 R C2 RERE  V EE Q1Q1 Q2Q2 Quiz

3. A differential amplifier will tend to reject a.noise that is in differential-mode. b.noise that is in common-mode. c.only high frequency noise. d.all noise. Quiz

4. The average of two input currents required to bias the first stage of an op-amp is called the a. input offset current. b. open-loop input current. c. feedback current. d. input bias current. Quiz

5. The slew rate illustrated is a. 0.5 V/  s b. 1.0 V/  s c. 2.0 V/  s d. 2.4 V/  s V out (V)  10   s Quiz

6. For the circuit shown, V f is approximately equal to a. V in b. V out c. ground. d. none of the above.  + Feedback network VfVf V in RfRf RiRi V out Quiz

7. For the inverting amplifier shown, the input impedance is closest to a. zero b. 10 k  c. 2 M  d. 8 G   + V in RfRf RiRi V out 150 k  10 k  Quiz

8. For the inverting amplifier shown, the output impedance is closest to a. zero b. 10 k  c. 150 k  d. 8 G   + V in RfRf RiRi V out 150 k  10 k  Quiz

9. The gain of the inverting amplifier shown is a.  1 b.  10 c.  15 d.  16  + V in RfRf RiRi V out 150 k  10 k  Quiz

10. A voltage follower has a. current gain. b. voltage gain. c. both of the above. d. none of the above. Quiz

Answers: 1. d 2. b 3. b 4. d 5. c 6. a 7. b 8. a 9. c 10. a Quiz