9/25/041 Elec and Comp Tech 62B Semiconductor Devices Chapter 10 Oscillators and Timers Feedback Oscillators

9/25/04 62Bchap10a Page 2 The Oscillator  The oscillator circuit produces a periodic waveform output with the power supply being the only required input  The output can be a sine, square, ramp, or other waveform  An additional input can be used to synchronize the oscillation  Two major classifications of oscillators  Feedback oscillators  Relaxation oscillators

9/25/04 62Bchap10a Page 3 Feedback Oscillator  Attenuation sets loop gain at 1  Net phase shift at oscillation frequency equals zero

9/25/04 62Bchap10a Page 4 Oscillator Start-Up  Some initial stimulus is needed to start oscillation  Start-up gain must be >1 for oscillation to build to desired level

9/25/04 62Bchap10a Page 5 Start-Up Stimulus  In a real oscillator circuit, noise from the op-amp is usually sufficient to provide the starting stimulus  In Multisim, some external stimulus is required to start the oscillation

9/25/04 62Bchap10a Page 6 Start-Up Gain  Gain much larger than 1  Starts oscillation with less stimulus  Oscillation builds to desired output voltage faster  Without some method of lowering the gain after reaching the desired output voltage, the output continues to grow until reaching amplifier saturation

9/25/04 62Bchap10a Page 7 Fixed-Gain Oscillators  Gain slightly greater than 1  May not start oscillating without external stimulus  Amplifier saturation is used to limit gain  When slight distortion is acceptable, additional gain limiting circuitry is not needed.

9/25/04 62Bchap10a Page 8 Wein-Bridge Oscillator  Bandpass filter  f r = 1/2πRC  Peak V out at f r V out /V in = 1/3  Feedback phase shift = 0

9/25/04 62Bchap10a Page 9 Wein-Bridge Oscillator  Wein bridge is created with lead- lag (bandpass) positive feedback and resistive negative feedback  Positive feedback V out /V in =1/3  Negative feedback sets A cl =3, so net gain through bridge = 1

9/25/04 62Bchap10a Page 10 Self-Starting Wein-Bridge  Start-up requires loop gain >3 to build output level  Continuous oscillation without saturation requires gain = 3  An automatic gain control (AGC) is needed

9/25/04 62Bchap10a Page 11 Back-to-Back Zener Method  At start-up, and as output builds, R 3 is in series with R 1, so A V >3  When voltage across R 3 >V Z +0.7 V, diodes short out drop across R 3, limiting gain to A V =3

9/25/04 62Bchap10a Page 12 JFET AGC  Q 1 operates in ohmic region  Negative peak detector D 1 and C 3 provide Q 1 bias to form voltage-variable resistance  R f is adjustable due to variances in g m of JFETs.

9/25/04 62Bchap10a Page 13 Phase-Shift Oscillator  3 RC hi-pass  f r = 1/2π 6RC when R 1, 2, 3 and C 1, 2, 3 are equal  When each individual RC phase shift (  ) = 60°, total network  = 180°, fed to inverting input, makes total  = 0°  Attenuation B (V out /V in ) = 1/29, so R f /R 3 =29 for unity gain of phase-shift network

9/25/04 62Bchap10a Page 14 Twin-T Oscillator  Hi-pass and Lo-pass T filters in parallel form a notch filter