# ramp generator- with an 741C as schmitt trigger

#### sean96

Joined Nov 5, 2018
25

here is an 741C configured as an schmitt trigger. i believe that the circuit is called a ramp generator. but dont quite know the working
of the circuit. how do i check the duty cycle of the schmitt trigger?? can i do that with D=PW/T? and i want to measure ramp voltage and time to workout the slope.dont a have much understanding about the operation of a ramp generator

#### AnalogKid

Joined Aug 1, 2013
10,943
You probably will get some grief for having a 741 (<gasp>) in the circuit. Ignore them. With a 15 V rail, the 741's output can go low enough to trigger the 555 reliably.

Whatever the input signal is (the symbol shows a sine wave, but who knows?), the 741 converts it to a square wave. First problem, if the signal swings both above and below ground, the below ground part will damage the 741 input stage. A more minor issue, I would increase the size of R1 (???), the 1K shunt resistor connect to 741 pin 3. The larger it is, the more hysteresis there is. Consider something around 4.7K to 10K.

The transistor is wired as a constant current source. R (value unknown) sets the current value. Working from the original capacitor equation, when a capacitor is charged up with a constant current, the voltage across the capacitor increases linearly. Pin 6 monitors the capacitor voltage. When it equals or exceeds 10 V, pin 7 discharges the capacitor to ground rapidly. The circuit then sits there until it is triggered again by the signal from the 741. The circuit makes a modified sawtooth wave, where the rising slope (the ramp) is much slower than the falling slope (the reset).

This is not a very good circuit, but it is ok for learning the fundamentals. Inside the 555 is a voltage divider network that creates internal reference voltages of 1/3 Vcc (5 V) and 2/3 Vcc (10 V). Pin 6 is the input to the threshold comparator that uses the 10 V reference.

You can measure the duty cycle with an oscilloscope at 741 pin 6. The duty cycle depends on the characteristics of the input signal, particularly the signal amplitude and any DC offset.

Please post a larger or more clear image, or a link to the original.

ak

Joined Mar 10, 2018
4,057
The 2N3906 functions as a current source driving a C. When you
drive a C this way yiu get a linear ramp.

Q = C x V
I = C x dV/dT or dV = ( I x dT ) / C in other words the voltage is a linear function
of I and the time I is flowing into C.

Here is a hysteresis calculator

Regards, Dana.

#### crutschow

Joined Mar 14, 2008
34,028
That single-transistor current-source current is sensitive to temperature and the sawtooth period is sensitive to the supply voltage.
Edit: I misread the circuit operation. The sawtooth period (duration) is not sensitive to supply voltage.
If you want a constant-current source for the ramp that minimizes these sensitivities, add a transistor to make a current mirror circuit.
Example below:
Although it adds a transistor, it has two fewer resistors.
For the best stability, you can use a matched transistor pair.

Last edited:

Joined Mar 10, 2018
4,057

#### AnalogKid

Joined Aug 1, 2013
10,943
the sawtooth period is sensitive to the supply voltage.
Actually, the waveform period is set by the signal input to the 741. The amplitude varies with Vcc. Replacing R(fuzzball) with a zener fixes this. Not perfect, but better.

ak

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#### sean96

Joined Nov 5, 2018
25
You probably will get some grief for having a 741 (<gasp>) in the circuit. Ignore them. With a 15 V rail, the 741's output can go low enough to trigger the 555 reliably.

Whatever the input signal is (the symbol shows a sine wave, but who knows?), the 741 converts it to a square wave. First problem, if the signal swings both above and below ground, the below ground part will damage the 741 input stage. A more minor issue, I would increase the size of R1 (???), the 1K shunt resistor connect to 741 pin 3. The larger it is, the more hysteresis there is. Consider something around 4.7K to 10K.

The transistor is wired as a constant current source. R (value unknown) sets the current value. Working from the original capacitor equation, when a capacitor is charged up with a constant current, the voltage across the capacitor increases linearly. Pin 6 monitors the capacitor voltage. When it equals or exceeds 10 V, pin 7 discharges the capacitor to ground rapidly. The circuit then sits there until it is triggered again by the signal from the 741. The circuit makes a modified sawtooth wave, where the rising slope (the ramp) is much slower than the falling slope (the reset).

This is not a very good circuit, but it is ok for learning the fundamentals. Inside the 555 is a voltage divider network that creates internal reference voltages of 1/3 Vcc (5 V) and 2/3 Vcc (10 V). Pin 6 is the input to the threshold comparator that uses the 10 V reference.

You can measure the duty cycle with an oscilloscope at 741 pin 6. The duty cycle depends on the characteristics of the input signal, particularly the signal amplitude and any DC offset.

Please post a larger or more clear image, or a link to the original.
how does having one power supply for the 741 IC effect the circuit.for example in the circuit pin 4 is grounded how does that effect the circuit?

Joined Mar 10, 2018
4,057
741 output swing very poor, with a 2K load, you have practically no load on 741
which makes it even worse.

Regards, Dana.

#### sean96

Joined Nov 5, 2018
25

#### AnalogKid

Joined Aug 1, 2013
10,943
how does having one power supply for the 741 IC effect the circuit.for example in the circuit pin 4 is grounded how does that effect the circuit?
In your circuit it has no effect at all. The circuit power supply 1s shown as 15 V. That means the Trigger input threshold level is 5 V. "As long as <whatever> is driving that pin can go below 5 V, the 555 will operate. With any single supply of 9 V or more the circuit will operate correctly.

Again, the 741 is fine for this application. Also, it's output stage voltage swing is the same as almost all opamps of its day (including some super low noise, high-fidelity wonders) and many opamps today. "Rail-to-rail" is the term for an improved output voltage swing performance, but the vast majority of contemporary parts are not that; they have the traditional 1 V to 3 V (per rail) headroom requirement of old.

ak

#### AnalogKid

Joined Aug 1, 2013
10,943
741 output swing very poor, with a 2K load, you have practically no load on 741
Your clipping does not show the power supply voltages for those output ranges. If they are +/-15 V, the National Semi 2000 datasheet has better 'typical' numbers.

Side note: As late as the mid-1980's, Hitachi 1" broadcast videotape machines had an audio system full of 741's. Noisy, but distortion and bandwidth weren't bad at all. After we shotgunned them with NE5534's the noise was WAY better, but the other specs barely moved.

ak

#### sean96

Joined Nov 5, 2018
25
In your circuit it has no effect at all. The circuit power supply 1s shown as 15 V. That means the Trigger input threshold level is 5 V. "As long as <whatever> is driving that pin can go below 5 V, the 555 will operate. With any single supply of 9 V or more the circuit will operate correctly.

Again, the 741 is fine for this application. Also, it's output stage voltage swing is the same as almost all opamps of its day (including some super low noise, high-fidelity wonders) and many opamps today. "Rail-to-rail" is the term for an improved output voltage swing performance, but the vast majority of contemporary parts are not that; they have the traditional 1 V to 3 V (per rail) headroom requirement of old.

ak
hmm i see

Joined Mar 10, 2018
4,057
Your clipping does not show the power supply voltages for those output ranges. If they are +/-15 V, the National Semi 2000 datasheet has better 'typical' numbers.

Side note: As late as the mid-1980's, Hitachi 1" broadcast videotape machines had an audio system full of 741's. Noisy, but distortion and bandwidth weren't bad at all. After we shotgunned them with NE5534's the noise was WAY better, but the other specs barely moved.

ak
I completely missed trigger is 1/3 Vcc. Shame on me. But note the trigger V in datasheet is a
typical spec. Because its based on a R ratio in chip probably fairly accurate. But then that
assumes Rs were not done with pinched off fets. Hard to tell.

Regards, Dana.

#### AnalogKid

Joined Aug 1, 2013
10,943
555 timer inventor -
Over the years I've met (briefly) most of the players who shaped the world we (circuit designers) live in. Hans was exactly what you would expect from that photo, one of the nicest guys in early IC design.

ak