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Ring Oscillator Schematic Circuit Diagram

Ring Oscillator Design: Transistor Amplifier Stages

In this configuration, the ring oscillator is structured with several inverting transistor amplifier stages linked consecutively, where the output of the last stage is linked back to the input of the initial stage. The flexibility is provided in the selection of stages, allowing for options like three, five, seven, or nine stages. However, a crucial condition is that the number of stages must be odd, precluding even numbers. Remarkably, this circuit operates without the need for capacitors, distinguishing it as a capacitor-free design. Such oscillators find extensive use in integrated circuits, notably in microcontrollers, showcasing their practical applicability.

Ring Oscillator Schematic Circuit Diagram 1

Ring Oscillator Schematic Circuit Diagram 2

Amplifier-based Oscillation with Negative Feedback

This oscillator functions fundamentally as an amplifier with negative feedback, achieving oscillation due to its high overall amplification. The depicted circuit utilizes five stages (as seen in Figure 1) to maintain the integrity of the ring. A buffer stage is strategically employed to decouple the oscillator signal, ensuring its stability. All resistors within the circuit carry a value of 2.2 kΩ, and the transistors used are BC548A types. Operating at voltages above 3 V, the oscillator generates a frequency exceeding 1 MHz, albeit slightly dependent on the power supply voltage, resulting in an average maximum frequency of 1650 kHz (refer to Figure 2).

Runtime Oscillation with Specific Signal Delays

This ring oscillator essentially functions as a runtime oscillator. The signal runtime across all five stages equates to half the oscillation period, precisely 300 ns at 1.65 MHz. Consequently, each individual stage exhibits a runtime of 60 ns. At higher voltages, the delay introduced by each stage increases slightly due to the transistors being heavily driven into saturation.

Diverse Applications in Chip Testing

A ring oscillator, illustrated in Figure 1, consists of an odd number (N) of inverting stages connected in series, with the output looped back to the input. This configuration allows for a mix of inverting and non-inverting stages, provided the total number of inverting stages remains odd.

Ring oscillators are invaluable during wafer testing, enabling the measurement of manufacturing process variations. Moreover, they serve as efficient tools for assessing the impact of voltage and temperature fluctuations on a chip’s performance. In practice, the oscillator achieves an average frequency of 33.45 MHz with a standard deviation of 0.88 MHz, operating at a supply voltage of 1.2V.

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