Engineering & TECHNOLOGY UNIVERSAL COLLEGE OF Engineering & TECHNOLOGY SUB:- ANALOG ELECTRONICS

Guided By:- Prof. Kapil Dave -:TOPIC:- CONCEPT OF NEGATIVE AND POSITIVE FEEDBACK Branch:- Electrical Engineering Div:- C Guided By:- Prof. Kapil Dave -:Prepared By:- Name En. No. Group no. Shardul s padhya 130460109032 10

CONTENT 1) 2) 3) 4) 5) Negative feedback 6) 7) Opamp circuit without Negative Feedback 8) 9) Op Amp Circuit Symbol and Terminals Op Amp Terminal Voltages and Currents Op Amp Circuit Model (Linear Region) Positive Feedback Op Amp Circuits with Negative Feedback What’s Inside an Op-Amp? Negative vs. Positive Feedback

OP AMP CIRCUIT SYMBOL AND TERMINALS V + positive power supply non-inverting input + – output inverting input V – negative power supply Figure 1 The output voltage can range from V – to V + (“rails”) The positive and negative power supply voltages do not have to be equal in magnitude (example: 0V and +3V DC supplies)

OP AMP TERMINAL VOLTAGES AND CURRENTS All voltages are referenced to a common node. Current reference directions are into the op amp. V + ic+ ip + – io + vp – in + vo – + vn – ic- V – – Vcc + + Vcc – common node (external to the op amp) Figure 2

OP AMP CIRCUIT MODEL (LINEAR REGION) Ri is the equivalent resistance “seen” at the input terminals, typically very large (>1MW, as large as 1012 W for FET input op-amps), so that the input current is usually very small: ip = –in  0 ip vp Ro io Ri vo + – A(vp–vn) in vn Note that significant output current (io) can flow when ip and in are negligible! Figure 3

POSITIVE FEEDBACK When we flip the polarization of the op-amp as shown on the figure we will get a positive feedback that saturates the amplifier output. This is not a good idea. Figure 4

POSITIVE FEEDBACK Whether the output is directly fed back to the inverting (-) input or coupled through a set of components, the effect is the same: the extremely high differential voltage gain of the op-amp will be "tamed" and the circuit will respond according to the dictates of the feedback "loop" connecting output to inverting input. Figure 5

POSITIVE FEEDBACK Another type of feedback, namely positive feedback, also finds application in op-amp circuits. Unlike negative feedback, where the output voltage is "fed back" to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the noninverting (+) input. In its simplest form, we could connect a straight piece of wire from output to noninverting input and see what happens:

POSITIVE FEEDBACK However, if there exist any anomalies in the waveform such as harmonics or "spikes" which cause the voltage to rise and fall significantly within the timespan of a single cycle, a comparator's output might switch states unexpectedly: Figure 6

NEGATIVE FEEDBACK Negative feedback is when the output is subtracted from the input. The use of negative feedback reduces the gain. Part of the output signal is taken back to the input with a negative sign. Figure 7

OP AMP OPERATION WITHOUT NEGATIVE FEEDBACK (COMPARATOR CIRCUITS FOR ANALOG-TO-DIGITAL SIGNAL CONVERSION) 1. Simple comparator with 1 Volt threshold:  V– is set to 0 Volts (logic “0”)  V+ is set to 2 Volts (logic “1”)  A = 100 +  V0 VIN 1V V0 VIN 1 2 If VIN > 1.01 V, V0 = 2V = Logic “1” If VIN < 0.99 V, V0 = 0V = Logic “0”

OP AMP CIRCUITS WITH NEGATIVE FEEDBACK Q: How do we know whether an op-amp is operating in the linear region? A: We don’t, a priori. Assume that the op-amp is operating in the linear region and solve for vo in the op-amp circuit. If the calculated value of vo is within the range from -Vcc to +Vcc, then the assumption of linear operation might be valid. We also need stability – usually assumed for negative feedback. If the calculated value of vo is greater than Vcc, then the assumption of linear operation was invalid, and the op-amp output voltage is saturated at Vcc. If the calculated value of vo is less than -Vcc, then the assumption of linear operation was invalid, and the op-amp output voltage is saturated at -Vcc.

NEGATIVE FEEDBACK Negative feedback is used to “linearize” a high-gain differential amplifier. With feedback Without feedback V0 +  VIN V0 +  V+ V V0(V) V0 5 5 4 3 2 1 VIN 1 2 3 4 5 5

OP-AMP FREQUENCY RESPONSE WITH AND WITHOUT NEGATIVE FEEDBACK Figure 9

OP-AMP FREQUENCY RESPONSE WITH AND WITHOUT NEGATIVE FEEDBACK Figure 10

OP-AMP FREQUENCY RESPONSE WITH AND WITHOUT NEGATIVE FEEDBACK Figure 11

NEGATIVE VS. POSITIVE FEEDBACK Familiar examples of negative feedback: Thermostat controlling room temperature Driver controlling direction of automobile Pupil diameter adjustment to light intensity Familiar examples of positive feedback: Microphone “squawk” in sound system Mechanical bi-stability in light switches

WHAT’S INSIDE AN OP-AMP? Figure 12

REFERENCE www.google.com www.wikipedia.com Robet.T. Paynter Introductory Electronic Device And Circuit Pearson Education

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