Analog Circuits. Operational Amplifiers (Opamps) DC Power Supplies Oscillators

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Julianna Maxwell
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1 Analog Circuits Operational Amplifiers (Opamps) DC Power Supplies Oscillators

2 Operational Amplifiers Highgain differential amplifier, using voltage feedback, providing stabilized voltage gain Symbol of differential amplifier inputs Inverting input (IN) Noninverting input (IN) output () dc power supplies Positive supply () Negative supply ()

3 Characteristics ery high input impedance (ideal infinite input resistance) ery small input currents (ideal ) ery high amplification (voltage gain) (A ideal ) Outut voltage bounded by power supplies values ery small difference between the absolute values of input voltages (ideal ) ery small output impedance (ideal zero output resistance) Two operation modes: differential mode: d IN IN common mode: c ( IN IN )/ (useful mode for rejecting noise signals); common mode rejection ratio CM parameter defined; expressed logarithmically in decibells db: CM[dB] log (A d /A c )

4 Negative eaction (feedback) An amount of the output signal is fed back to circuit s input, with a phase shift of 8 from input signal, implying a decrease of the output signal IN eactie negativa

5 ConstantGain OpAmp IN I I I IN IN

6 Noninverting OpAmp IN IN I I IN ( ) IN

7 I I I n Summing OpAmp IN IN I IN... (... IN n INn n INn ) INnn

8 Integrator Z C j ω C sc IN C I I C Z IN I sc C sc IN

9 Differentiator Z C j ω C sc IN C I I C Z IN I C sc IN sc IN

10 Power Supply Circuits Convert analog supply voltage into desired regulated dc voltage Circuit consists on: transformer: separates circuit from ac power supply network and steps input ac voltage to a desired ac level, conform to the needed dc voltage level to be obtained rectifier: to halfwave or fullwave rectify the ac signal, getting a dc component non zero filter: attenuation of ac voltage irregularities (as coming from rectifier) voltage regulator: regulates the filtered dc voltage to a desired value for output voltage

11 HalfWave ectifier D is open for positive polarities and off for negative polarities of the applied input voltage

12 FullWave Bridge ectifier Four diodes in a bridge configuration D and D are open for positive waves D and D 4 are open for negative waves of applied input voltage

13 ectifier with filtered output A capacitor C in parallel with load resistance L If rectifier provides a higher voltage than the voltage drop on capacitance, it stores energy If rectifier provides a lower voltage than capacitor s voltage drop, C will generate energy

14 oltage egulators Zener diode shunt regulator Output voltage must stay fixed, for different loads ( L values) Filtered output voltage has a dc component and some ac variations (ripple voltage) Zener diode shunt regulator based on nonlinearity of the currentvoltage characteristics for a Zener diode Allows for high variations of output currents, for small variations of diode s reverse voltage drop

15 oltage regulator with negative feedback and without error amplifier Negative feedback from collector to base circuit of Q Output voltage (v ) compared with a reference voltage (v Z ) comparison generates an error signal applied to a control element if v increases, control element (negative reaction) will impose voltage decrease, and viceversa For transistor Q: v BE v Z v if v goes higher > v BE lows > v CE higher > v lower if v lower > v BE higher > v CE lower > v higher v v Z v BE D Q D D D4 C m Z D5 L

16 oltage regulator with feedback and error amplifier v v if v lower > v lower > v higher > v BE higher > v CE lower> v higher if v higher > v lower > v lower > v BE lower > v CE higher> v lower Because D v v Z v Q v Z D D D4 C m Z D5 k k L

Integrated Circuits (IC egulators) 78 series provides positive output regulated voltages 79 series provides negative output regulated voltages Last two

17 Integrated Circuits (IC egulators) 78 series provides positive output regulated voltages 79 series provides negative output regulated voltages Last two digits from code show regulated voltage value Input capacitance avoids oscillations due to stray inductances of supply line Output capacitance acts as filter

18 Oscillators Generates a repetitive waveform, without any input stimulus Amplifier and positive feedback Positive feedback: an amount of output voltage is fed back to input with zero phase shift, providing a gain of the output voltage Conditions for having oscillations: Total phase shift must be zero oltage gain must be greater than for obtaining desired oscillations amplitude. After gaining desired amplitude for oscillations, voltage amplification must stay at Amplificator eactie pozitiva

19 Wien bridge Oscillator Positive feedback circuit, made up by an opamp and C bridge circuit Wien bridge C highpass and C lowpass filters Each filter operates over edge frequencies (frequency adjustment), C C C attenuation is minimum for oscillation frequency f osc πc IN IN C C

20 Base circuit Opamp with negative feedback For having a unit voltage gain for oscillator, amplification for base circuit must be IN For lower oscillation amplitudes the two Zener diodes are off (no direct diode current) IN When having desired amplitude, the diodes go conducting, shorting and giving a voltage gain for the base circuit of, and a voltage gain for oscillator of D D

21 D D K K K C 5 4 K n C K n Wien bridge oscillator

22 Problem Design a current source, being voltage driven AO repeater For calculus of superposition paradigm is used I I I S S AO 4 AO S 5

23 I I S S ) ( ) ( I S must depend only on coefficient must be I S ) ( AO 4 AO S 5

Operational Amplifiers

Operational Amplifiers Fundamentals of op-amp Operation modes Golden rules of op-amp Op-amp circuits Inverting & non-inverting amplifier Unity follower, integrator & differentiator Introduction An operational amplifier, or op-amp,