Transistor amplifier circuit for triangular wave


I've made triangular wave oscilator as shown in Fig. 1 and it works perfectly at frequancy 10 kHz with peak voltage +3 Volt to -3 Volt.

Figure 1. Oscilator circuit

I intend to generate SPWM. But when I feed this circuit to comparator, the output drops to 0 Volt. How can this happen?

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I assume that output current of op-amp IC TL-082 is very small, not enough to feed comparator. If my assumption is correct, can I use transistor amplifier to produce enough current?

Best Answer

You say:

I use TL082 as comparator with negative supply (-Vcc) connected to ground directly.

In the datasheet for the TL082, under APPLICATION HINTS, it clearly states:

However, neither of the input voltages should be allowed to exceed the negative supply as this will cause large currents to flow which can result in a destroyed unit.

Your integrating opamp, IC(2/2), is using a bipolar supply, and the output signal swings both positive and negative. However, when you connect this signal to the third opamp, which has VEE connected to ground, the signal gets clamped at ground. This is because when it tries to go below ground, the third opamp allows a "large current to flow" (i.e., its input impedance drops dramatically). This stops the oscillation, because now the first opamp, IC(1/2), can't switch at -3V like it normally does.

The solution is to connect VEE of the third opamp to the same negative supply as the other opamps. Why didn't you do this to begin with?