ECE Lab 2, 3, 4, 5 & 6: Op-Amps I & II, Filters I & II, Diode Circuits

Operational amplifier as a Comparator

Active device: capable of supplying energy to a circuit.

Needs external power supply.

Can modify/ amplify circuit.

High input impedance

Little current in/out of V+ and V- pins.

Doesn’t present load on preceding circuit elements.

Low output impedance.

Source/ sink current from output → provide output to next elements

Provide Buffering or Isolation b/w circuits

Op-amp Open-Loop Gain

Inverting Amplifier

Op-amp with negative feedback.

Multiplies the input voltage by a negative gain, producing an output signal that is 180∘ out of phase with the input

180 Degree Phase shift = multiplying by factor of -1

Inverting Summer

Three inputs

Non-Inverting Op-amp

Amplifies input voltage without phase shift.

JFET as a variable resistor

Voltage-controlled resisstor

As V_c goes from 0V → V_th

R_th goes to inffinity

Filter

The fundamental purpose is to change the magnitude and phase of the sinusoidal components of an input signal.

Filtering

Manipulating Periodic Signals based on their frequency content.

Periodic signal - Fourier’s theorem

A periodic signal may be represented by a sum of sinusoidal components.

Frequency Domain and Sine waveform

Voltage vs. Frequency

Plot of the magnitude of sinusoidal components vs. their frequency.

A pure sinusoid has all signal power concentrated at a single frequency component (a delta function). Amplitude = V_pp / 2

FFT - Fast Fourier Transform analyzes a signal's time-domain waveform and converts it into the frequency domain.

Square Waveform

Amplitude = 4 / π n

(n = 1 for the fundamental frequency)

Frequency Response H(jω)

ω = 2 π f

Active first-order low-pass filter

Active first-order high-pass filter

Advantage of active filter

Property of Op-amp making it possible

Allows us to add a load without changing gain

It isolates the load resistor

Active second-order low-pass filter

Provide improved performance in signal processing.

f_c = x f_0

Active second-order high-pass filter

f_c = x f_0

Quality Factor

Influences Shape of the frequency response

Increasing Q leads to a sharper/ narrower peak near the resonant frequency and a higher peak Gain

Critically Damped: Q = 1/sqrt(2) and f_c = f_0

Diode

Two terminal circuit element that has polarity, current can only flow in one direction.

Forward Bias: 0.6V IDEALLY 0V (acts as a wire)

Reverse Bias: OPEN

Peak-Hold Circuit / Peak detector circuit

Monitors positive/ negative peak of an input signal.

V_OUT = V_IN when the input signal is increasing. 

If the input signal drops below the peak value the output will hold.

Diode Input Stage (non-linear) → (U1, D1, D2)

Voltage follows the input as it increase

If V_IN drops below peak, Diode 1 will reverse bias preventing V_A from dropping

Stage 2 → (R1, C1)

The capacitor (C1) holds the peak voltage by storing charge.

The resistor (R1) provides a discharge path to ground.

Time Constant

Amount of time it takes the output to decay by a factor of 1/e = 0.368

t = R1C1

Voltage Follower Stage → (U2)

Buffer that prevents the scope from loading and prematurely discharging the capacitor.

LED Graph Bar module

Allows us to illuminate a sequence of 10 LEDs in proportion to a single input voltage.

Oscillator Circuit

Produces a periodic output signal without any applied input signal

Schmitt trigger circuit

Comparator + Positive feedback = Hysterisis (2 thresholds)

"memory" effect makes the circuit less sensitive to noise.

Used to convert a noisy input signal into a clean digital output.

Relaxation Oscillator

Schmitt Trigger + RC Circuit

R1, R2, R3 and C all determine the frequency of the oscillation

Square waves

Linear DC Power Supply Circuit

1. Mains Voltage (120 VAC, 60 Hz) Standard Wall Outlet

2. Step Down Transformer Wall Wart: Steps down 120VAC to safer 9 VAC, 60 Hz.

3. Bridge Rectifier: 4 diodes to convert the bipolar AC sine wave into a unipolar (positive only), 9 VAC,120 Hz Doubling the number of peaks per second.

4. Filter Capacitor: Smoothens the pulsating DC into a "ripply" DC voltage by storing charge and releasing it between the peaks.

5. Voltage Regulator: Rremoves nearly all the remaining ripple to produce a clean, stable DC output.

AC Power module

mm audio jack adapter and pin comnnections

Pulse Width Modulation

A pulse width modulation signal encodes information in the width of a train of regularly spaced square pulses.

NOt Amplitude.

Used in Motor control, Analog to discrete conversion and voltage regulators.

Sample rate: frequency at reach sample is taken

Either “low” or “high”

The higher the input amplitude, the longer the PWM is high

Duty Cycle: Percent of time PWM is high for a sample period

LM311 Comparator

Not Op-amp

Open collector output: Can’t output supply voltages on its own

Pull-up Resistor R_p well defines COL_OUT terminal as +Vs

Pulls up PWM output so it doesn’t float/ fall flat.

Class-D Amplifiers

Uses Rapid ON/OFF switching of transistors instead of a constant  bias current. 

G = V_OUT/ V_IN

Complementary MOSFET Switching

Logically Invert PWM_IN Signal creating PWM_MOD

Voltage controlled switches

Opposite polarity: when P-MOSFET is on and pulls PWM_MOD to +Vs, N-MOSFET is off

Switches and Sensors

Op-amps: current limited outputs

Physical Relay: electromechanical switch normally open

MOSFET: Solid State Voltage controlled switch 

Push Button: Physical switch get’s shorted

Schmitt Trigger: “de-bouncing” Hysteresis for clean switching.

Photocell: Resistance increases as light reduces

Temperature sensor voltage increases as temperature increases

LED circuit with automatic ON/OFF

Light decreases, resistance increases, U1+ increases → +Vs

Capacitor charges, discharges through push button

U2 is a voltage follower, buffer, provides current to relay coil.