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Controlled Amplitude High Frequency Oscillator

IP.com Disclosure Number: IPCOM000082652D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Malaviya, SD: AUTHOR

Abstract

Shown is a crystal-controlled oscillator for use in stable timing and clock circuits.

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Controlled Amplitude High Frequency Oscillator

Shown is a crystal-controlled oscillator for use in stable timing and clock circuits.

Transistors Q3 and Q4 constitute a current switch whose emitters are AC coupled by crystal XTAL. The output of Q3 is buffered by Q1 and fed to Q4, and the output of Q4 is buffered by Q2 and fed to Q3. Resistors R3 and R4 provide DC negative feedback, which tends to stabilize the DC operating points of Q3 and Q4. Diode D1 and resistor R6 also provide a DC path for buffers Q1 and Q2. Resistor R6 and D1, with resistors R3 and R4, form a mixer circuit M.

The outputs of Q3 and Q4 are fed back to inputs of Q4 and Q3, respectively, via emitter-followers Q1 and Q2 to provide a positive feedback. The same outputs are also fed back to their own inputs to provide negative feedback. For example, the output of Q3 is available at the emitter of Q1 and is fed back to the base of Q3 via R4 and R3, thus providing negative feedback. A similar situation exists for Q4. R5 and R7 provide DC bias to transistors Q3 and Q4.

When the amplitude of oscillations is small, the increase in current in R3 is equal to the decrease in current in R4 and vice versa, so that the net current in D1 remains constant. This means that the voltage at node A (junction of R4 and R3) remains constant and there is no negative feedback from the emitter of Q1 to the base of Q3, or from the emitter of Q2 to the base of Q4. However, strong positive feedback exists from the emitter o...