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Programmable Function Generator

IP.com Disclosure Number: IPCOM000089645D
Original Publication Date: 1977-Dec-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Bigbie, SE: AUTHOR [+2]

Abstract

The circuit of Fig. 1 comprises a comparator with a switched reference level. This distinguishes from other available circuits which, for example, use two comparators and two reference voltages. Also, it may be placed with other circuits on an integrated circuit chip. In the circuit, the frequency, period, and duty cycle are absolutely constant and completely independent of power supply voltage levels and transistor electrical parameters. More than one function may be performed.

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Programmable Function Generator

The circuit of Fig. 1 comprises a comparator with a switched reference level. This distinguishes from other available circuits which, for example, use two comparators and two reference voltages. Also, it may be placed with other circuits on an integrated circuit chip. In the circuit, the frequency, period, and duty cycle are absolutely constant and completely independent of power supply voltage levels and transistor electrical parameters. More than one function may be performed.

Resistor R3 and transistors Q10 and Q11 form a constant current source to the differential amplifier comprising transistors Q2, Q3, Q4, Q5. They are not involved in wave generation functions. Transistor Q9 is the output transistor. One feedback path is from the collector of transistor Q8 to the base of transistor Q1. A second feedback loop exists from the collector of transistor Q8 to the node of V-Ref (base of transistor Q5) through transistor Q6 to cause oscillation.

Assume transistor Q1 is turned on; its collector discharges the capacitor C to the level of V-Ref-Down at the base of transistor Q5. This results in transistors Q2/Q3 turning off. The current is now steered to transistors Q4/Q5, which turn on, and with them, transistors Q7, Q8 are also turned on. The collector of transistor Q8 is now at ground level, and this opens up the collector of transistor Q1. Now capacitor C starts charging up from the power supply through R1 and R2. When its level reaches V-Ref-Up at the base of transistor Q5, it turns on transistors Q2/Q3. This shuts off transistors Q4/Q5, transistor Q7 and transistor Q8. Now the collector of transistor Q8 is high, which turns transistor Q1 on, and capacitor C starts discharging. The cycle repeats itself and a continuous wave is generated.

When capacitor C discharges, the feedback loop to transistor Q6 is interrupted, opening the Q6 collector, and R6 is left "hanging". The reference voltage at the base of transistor Q5 is now "V-Ref-Up". Conversely, when capacitor C starts charging up, the feedback loop from transistor Q8 turns on transistor Q6 to saturation. Since the saturation voltage from collector to emitter of transistor Q6 is only a few millivolts, resistor R6 is considered as returned to ground. Resistor R6 is now in parallel with resistor R7, and the effective value of the combination is less than resistor R7...