Ring oscillator-1

Ring oscillator, which is a kind of voltage-controlled oscillator, is a very common topology in current technology. It can be used in many different fields, such as Analog Design, Digital Design, and RF design. In Figure 1, this topology is combined by three or odd numbers of inverters. The frequency of inverters can be influenced by increasing numbers of inverters, adding capacitors or adjusting the size of MOSFET. 

Figure 1. Topology of Ring Oscillator

The reason why this topology oscillates is gate delay. In the real world, because the gate (inverter) can't change the status immediately. Every gates need time to change the status. In odd numbers of inverters, if the last output of inverter is 1, output of first inverter will be -1 and cause oscillate.  Therefore, adding capacitor is useful for changing the time delay and its frequency. The formula of oscillation is given by:

\[f=\frac{1}{2*t*n}\]

However, in this topology, the numbers of inverters can be three or even more. What can we do if we only want use two blocks in the oscillator? Using operational transconductance amplifier (OTA) may be a good option.  OTA is the device, which can transfer input voltage to output current. Like Operation Amplifier, they are open circuit. Therefore, they usually add a feedback circuit in the real design. OTA has higher input resistance than Operation Amplifier. Figure 2 is the topology of operational transconductance amplifier. In this design, there are two output currents, and delay 180 degree.  

Figure 2. Topology of operational transconductance amplifier

Figure 3 is the design of Ring oscillator based on OTA. Because OTA can output current in the same time. Therefore, is very easy to oscillate. As other Ring oscillator, it can be controlled by voltage, capacitor, and numbers of blocks.

Figure 3. Ring Oscillator based on OTA