Exponential Voltage - Frequency Converter for Electronic Music

In an electronic music synthesizers require voltage controlled frequency generators whose output frequencies are exponentially related to the input voltages. This is one solution for the problem. This is a circuit for convert voltage to frequency. This is the figure of the circuit.


This circuit has performs this function with 0.25% exponential conformity over a range from 20 Hz to 15 kHz using a single LM392 and an LM3045 transistor array. The exponential function is generated by Q1, whose collector current will vary exponentially with its base-emitter voltage in accordance with the well known relationship between BE voltage and collector current in bipolar transistors. Normally, this transistor's operating point will vary wildly with temperature and elaborate and expensive compensation is required. In this circuit, Q1 is part of an LM3045 transistor array. Q2 and Q3, located in the array, serve as a heater sensor pair for A1, which servo controls the temperature of Q2. This causes the entire LM3045 array to be at constant temperature, eliminating thermal drift problems in Q1's operation. Q4 acts as a clamp, preventing servo lock-up during circuit start-up.