# Creating a Voltage Controlled Oscillator without using IC

#### Fatih Çalış

Joined Dec 3, 2016
15
In Analog Laboratory course, I am expected to create a VCO, with only passive elements, opamps, transistors, diodes. The VCO is to be working in between 1kHz and 5kHz. The only signal I can use from the signal generator is a triangular wave with 10Hz.
Does anyone have any clue how to create a VCO with controllable frequency oscillation which works in these frequencies?

#### DickCappels

Joined Aug 21, 2008
7,743
Have you tied including the words "discreet components" in your Google search?

#### MrAl

Joined Jun 17, 2014
8,481
In Analog Laboratory course, I am expected to create a VCO, with only passive elements, opamps, transistors, diodes. The VCO is to be working in between 1kHz and 5kHz. The only signal I can use from the signal generator is a triangular wave with 10Hz.
Does anyone have any clue how to create a VCO with controllable frequency oscillation which works in these frequencies?
Hi,

An op amp is made from transistors, you can make a VCO from an op amp, therefore using transistors you can create a VCO.

Now obviously you probably dont want to have to recreate the entire op amp with associated multiple transistors, so try to figure out the min number of transistors you would need to mimic an op amp and then build the VCO from that.

You can make a VCO from an op amp or comparator with four resistors and one capacitor. You should be able to do the same with one or two transistors maybe a couple more resistors, but give it some thought.

#### Bordodynov

Joined May 20, 2015
2,909
For entertainment, I made the necessary circuit consisting of 4 transistors, 3 resistors 1 capacitor.

#### MrAl

Joined Jun 17, 2014
8,481
For entertainment, I made the necessary circuit consisting of 4 transistors, 3 resistors 1 capacitor.
Hi,

I did one a long long time ago i think it used four or five transistors too, but i feel it was too complicated to be practical for this thread. It was just for the fun of doing it too.

#### Bordodynov

Joined May 20, 2015
2,909
I searched the Internet for a pulse generator on an analog of a unijunction transistor. I turned over the circuit. I also used a current mirror. It's much easier. Using LTspice, I picked up the values of the elements.