Op-Amps Oscillate
by Darren Ashby

Every time I write about op-amps, I get an email from someone who needs help with an op-amp design. So I have to believe that this is something my readers would like to know. One very handy thing that op-amps can do is to oscillate. I canęt say that it is handy all the time. I have designed circuits that did oscillate exactly when I didnęt want them to. So I thought we might discuss how to make an oscillator, and how to make it stop. To start with, I will NOT use an op-amp. I will use a logic chip that any digital engineer worth his salt will recognize, the 74HC14 Schmidt trigger hex inverter.

What is the hex inverter? Well, hex means a 6 pack, and inverter means a high on the input will be inverted to a low on the output. Now what about the Schmidt trigger? This indicates that the output will not switch until the input crosses an upper or lower threshold. For example, if there is an upper threshold of 1.6V, then the output will remain high for an input of 1V and will go low for an input of 2V. The reverse is true also. The output will not go high until the input goes below the lower threshold. This whole notion is known as hysteresis, and it is caused by positive feedback. This basically creates a no-man's land where the signal will not change its current state. It is great for rejection of noise on a signal, but there is something else that it can do. So pull out a 6 pack of inverters with hysteresis and hook one of them up like this.

Figure 1: Simple Square Wave Oscillator

Figure 2: Graph of Input (black) and Output (red)
of a Square Wave Oscillator

When the output goes high, the input charges up until the output trips and then discharges until the opposite happens. So how does this apply to an op-amp oscillator? Well letęs see if we can make one based on this model. We need an inverter first. Thatęs easy. We use the inverting input. Then we need some hysteresis, which will take a couple of resistors and positive feedback. Then we tie the output into the input and presto!

Figure 3: Op-amp Oscillator

Please note that the 3 resistors in the positive feedback network control the threshold levels that were built into the 74HC14. But these are the same types of oscillators. I would like you to note one thing that is true in both cases. To oscillate typically requires negative and positive feedback.

Do you want to change the duty cycle of the square wave? If so, try this.

Figure 4: Op-amp Oscillator with a Changed Duty Cycle

Now one negative feedback resistor controls the charge time of the cap and the other controls the discharge time.ę You can add a circuit like this to create a triangle wave output.

Figure 5: Triangle Wave Oscillator

Basically what you are doing is putting the square wave into an integration circuit. And the integral of a square wave is a triangle wave.

Now, if you want a sine wave, you can add some more filtering to the above circuit to get a sine-like signal, or you can try this circuit that a reader sent in with a question. He needed a sine wave signal with precise amplitude.

Figure 6: Sine Wave Oscillator

He couldnęt get it to work, and questioned the need for the zener diodes in the feedback loop. The first thing I noticed was that there is a stable condition when the inputs are exactly equal to each other and the output is equal to zero. However, this is unlikely to happen in the real world. So I discounted it as a problem. In this circuit, the positive feedback network creates the sine shape. And I realized that the zener diodes were very important. They are what cause the output to switch directions. Think about it. The output is trying to make V- = V+ (if you doubt this read my archives on negative feedback) as Vout exceeds the threshold of the diodes at the peak of the sine wave, this dumps current into V-.ę To compensate for this, Vout starts going down where the process is reversed at the trough of the sine wave. Another critical item I found was the ratio of the negative input resistor to the negative feedback resistor. If it is greater than ę the output tends to dampen out and stop oscillating. So I recommended that the negative input resistor be slightly less that ę the negative feedback resistor. It is also necessary that the resistors in the positive feedback network are precise. But all this just got the circuit oscillating. And what was required was precise amplitude. This got me thinking. I realized that the amplitude is dependent on the zener diode voltage. If I could some how adjust the diodes I would have it. I noodled around for a while when the light bulb finally came on. Donęt adjust the diodes, adjust how much voltage they get. After that it was easy. So here I present my adjustable amplitude sine wave oscillator.

Figure 7: Adjustable Amplitude Sine Wave Oscillator

Please note that the resistors should be +/-1% as the circuit is sensitive to minor variations. The potentiometer needs to be less than the negative input resistor to have a significant enough affect on the feed back loop to make the amplitude adjustable. If the only pot you have is larger, you can change the negative feedback and input resistors. But the ratio must remain the same for it to work well. If you canęt find a 1.49K resistor try putting a 1 Meg resistor in parallel with a 1.5K. Remember that it should be slightly smaller than ę of the negative feedback resistor.

Well there you have it, some circuits that you want to oscillate. Some day we will talk about oscillations you donęt want.

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