Project Guide
 Apurva S
IAS chair
Mentors

Venugopal R

Harshak Sachdeva

Kaushik Srivastava
Members

Adithya Kasal

Aditya Raj Pandia

Gururaj Vijay Shimpi

Vaibhav Santhosh
Acknowledgements
We would like to thank the envision team and seniors of diode for their constant support throughout the project.
Introduction
Analog music synthesis, renowned for its warm and organic sound, continues to captivate musicians despite the rise of digital technologies. Central to Analog synthesis is the voltagecontrolled oscillator (VCO), which generates fundamental waveforms and enables precise control over pitch through voltage modulation. This project aims to design and implement a VCO using bipolar junction transistors (BJTs) and operational amplifiers (opamps). The objective is to develop a VCO circuit that accurately translates linear voltage changes to logarithmic frequency perception, aligning with human auditory expectations, using a linear to exponential voltage converter and a VCO. This project combines theoretical analysis and circuit simulation to assess the VCO’s performance in terms of frequency linearity, stability, and tuning range.
Linear to Exponential Voltage converter using Differential Amplifier
Why do we need a linear to exponential voltage converter
A lineartoexponential converter is an essential part of many music synthesizers, as it provides a way to convert a linear voltage input into an exponential voltage output. This allows one to control various aspects of the sound produced by the synthesizers, such as pitch, amplitude, and filter cutoff frequency, in a more intuitive way. The pitch of a sound is typically controlled by a voltagecontrolled oscillator (VCO), which generates a waveform at a frequency proportional to the input voltage. However, because the human ear perceives pitch logarithmically, a linear voltage input does not result in a linear change in pitch. To solve this problem, a linear to exponential converter is used to convert the linear voltage input into an exponential voltage output, which is then used to control the frequency of the VCO. By applying 5 volts of linear input to the converter, the resulting exponential output voltage can be used to control the pitch of the VCO in a more musically meaningful way.
Amplifier circuit
The specific circuit used for the lineartoexponential converter will depend on the specific requirements of the synthesizer. However, a basic circuit may consist of a differential amplifier and an opamp. The input voltage is applied to the noninverting input of the differential amplifier, while the inverting input is grounded.
The output of the differential amplifier is then fed into the noninverting input of the opamp and the output of the opamp is the exponential voltage output. The output voltage can then be used to control the frequency of the VCO using a voltagecontrolled filter or other voltagecontrolled modules in the synthesizer. By adjusting the input voltage, one can create a variety of sounds with different pitches and dynamic characteristics. Each input voltage corresponds to a different note in the musical scale.
Temperature variation
The reasoning behind why we use two BJTs in the design is that we need to eliminate the variation in the output due to changes in temperature. This can be done by matching the two transistors exactly such that their parameters such as gain vary similarly with change in temperature. To deal with this variation, we use two perfectly matched transistors as shown. Both of them need to have the same, or nearly same, temperature coefficients and gains.
Voltage Controlled Oscillator
What is a VCO?
A Voltage Controlled Oscillator (VCO) is an electronic oscillator that generates a waveform whose frequency can be controlled by a voltage input. The output frequency of the oscillator is determined by an input voltage that controls the tuning of the oscillator circuit. The VCO is a fundamental building block in many electronic systems, including radios, synthesizers, and communication systems. VCOs are available in various types, such as voltagetofrequency converters, relaxation oscillators, and crystal oscillators. They can operate at different frequencies ranging from a few Hertz to several gigahertz, depending on the application.
###Circuit Diagram of VCO
The circuit used in the project takes in the input voltage (coming from the linear to exponential voltage converter) and generates a triangular waveform and a square waveform. The triangular waveform is generated using an integrator circuit and the square waveform is generated using an inverting Schmitt Trigger
Working of the VCO
As the input is fed into the integrator, the output waveform is triangular. The capacitor gets charged in this process. The generated triangular waveform is then given as input to the Schmitt Trigger. As long as the voltage doesn’t go beyond the upper threshold voltage of the Schmitt Trigger, the second output is +Vsat (Saturation Voltage). Due to this, the baseemitter junction of the transistor is in forward bias and the circuit is complete. The instant it crosses the upper threshold voltage value, second output is Vsat. This causes the baseemitter junction of the transistor to become reversedbiased and break the circuit. Now, the capacitor discharges, and the output 1 drops linearly. This continues until the voltage drops below the lower threshold voltage and the 2^nd^ output is again +Vsat. Alternating +Vsat and Vsat gives the square waveform as the output, which is given to the speaker.
Simulation and Result
The circuit shown above is the one that was simulated for the project. It utilizes all the components discussed previously including the integrator, differential amplifier, Schmitt trigger, and others. The simulation was done in LTSpice.
First input signals Key, Tune, and LFO were sent into the adder OpAmp and then led to the differential amplifier. Following the linear to exponential conversion, the signal was sent as input to the VCO. At the VCO, the two waveforms, square and triangle, is the output of the synthesizer.
Conclusion
In conclusion, this project has successfully designed and implemented a voltagecontrolled oscillator (VCO) using bipolar junction transistors (BJTs) and operational amplifiers (opamps). By incorporating a linear to exponential voltage converter, the VCO achieved accurate frequency control that aligns with human auditory perception. The comprehensive analysis confirmed the reliability and precision of the VCO’s performance, meeting the requirements for realworld musical applications.
Each member involved in this project learned the working of BJTs, OpAmps, and their application circuits. This gives them a solid foundation in analysing Analog electronic circuits and designing them.
References
1.1V/octave VCO, Link
2.Voltage controlled oscillator, Link