3B1 VCVS & Filters & Oscillators

Wein Bridge oscillator circuit schematic?

Describe what Colpitts oscillator is?

And frequencies that can be achieved?

LC resonant circuit with transistor as the active element driving the oscillations

Frequencies up to several GHz achieved

What makes an op-amp ideal?

No current drawn by the inputs

How to make high-pass filter VCVS?

And how to make bandpass?

High-pass: switch Rs and Cs from low-pass diagram

Bandpass: cascade overlapping low-pass and high-pass stages

Low-pass 2 pole single stage VCVS filter circuit schematic?

Low-pass 2 pole, 2stage VCVS filter circuit schematic?

Standard E12 values?

1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2

What is f_c?

cut-off (-3dB) freq

High-pass 2 pole, 2stage VCVS filter circuit schematic?

What are poles in filter design?

Frequencies where the gain drops sharply

(when transfer function’s denominator goes to zero)

Each opamp creates 2 poles

Natural frequencies from how R & C interact in each stage

High-pass f_c and gains in 4 pole VCVS?

• Swap Rs and Cs → f_n becomes 1/f_n

• OpAmp gains are same for poles 1&3 and 2&4

Negative impedance oscillator circuit schematic?

When to choose each of the VCVS filters?

Bessel: e.g. shaped electrical pulses

fixed time delay, no overshoot, good time domain response

↑ best for passing transients undistorted

PULSE SHAPE RETENTION BUT REDUCED SHARPNESS

Chebyshev: steep cut-off

best frequency cut-off but distorts waveform shapes

Butterworth: reasonably good cut-off without significant overshoot

a fair all rounder

compromise between the other two

Should you pick higher or lower R & C standard values in low-pass, high-pass and bandpass filters?

How does a negative impedance oscillator work?

by cancelling the resistive losses in an LC circuit with a transistor to create sustained oscillations

Steps to draw -ve impedance oscillator SSM?

• draw both transistors

• connect Es

• first B & second C to ground

• R_T from common E to ground

• R_b from common C/B to ground

• Vo above both transistors (with i_o going down into common B/C)

• C_b dashed in common C/B

Main result from -ve impedance oscillator SSM?

Z_o = R_b // (-2 r_e)

Steps to find -ve impedance oscillator result?

Why do we want to add a resistance to -ve impedance oscillator?

to cancel out resistive parasitic component of LC combination

Circuit schematic of -ve impedance oscillator with LC resonant tank circuit?

How to find parasitic loss resistance of -ve impedance oscillator?

wLQ

[Q = Q factor]

Rule to find Rb from inductor parasitic R & -ve transistor R?

Want overall resistance to be -ve and less than 0.5 RL, but same order of magnitude

Ic in -ve impedance oscillator?

Ic = ½ (5 - 0.6) / R_T