How To Measure The Switching Voltage And The Hysteresis Of A Cmos Gate

This little circuit lets you measure both the switching voltage - and the hysteresis - of common Cmos inverters and non-inverters such as 4001, 4011, 4093, 4071, 4081.

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Learn More About Cmos Inverters

A Cmos ICs will work at anything from 3v to 15v - some versions will even work with an 18v supply. A Cmos input pin is high when it's more than half of the supply voltage - and low when it's less than half of the supply voltage. Roughly speaking - with a 12v supply - anything over 7v is a high - and anything below 5v is a low. Somewhere between 5v and 7v is the crossover - or switching point of the gate. Depending on whether the voltage on the input pin is rising or falling - when it crosses this point - the output pin will switch from high to low - or vice-versa.

If you're designing timers - thermostats - light operated switches etc - the precise switching voltage of the Cmos gate may be important. For example - in the case of an RC timer - it will take the capacitor longer to charge to 7v - than it will to charge to 5v. So the actual time delay produced by any particular RC pair - will be acutely dependent on the switching voltage of the gate you're using. This page provides two small circuits that will allow you to measure the exact switching voltage of common Cmos inverters and non-inverters.

Circuit No.1 - Inverters
	In the case of inverters - the state of the output pin is always opposite to that of the input pins. That's why they're called inverters. When the input pins are high - the output pin will be low. And when the input pins are low - the output pin will be high. ============ An input pin is high when it's more than half of the supply voltage - and low when it's less than half of the supply voltage. Roughly speaking - with the 12v supply -anything over 7v is a high - and anything below 5v is a low. Somewhere between 5v and 7v is the crossover - or switching point - of the inverter. Depending on whether the voltage is rising or falling - when it crosses this point - the output pin will switch from high to low - or vice-versa. ============ The circuit on the left should work for most common inverters. But the connections in the drawing are for the Quad Two-Input variety. These are the 4001 - the 4011 - and the 4093. ============ You only need to measure the voltage on one of the gates. All four gates were manufactured at the same time - using the same materials. So they should all have the same characteristics. ============ The circuit works by repeatedly charging and discharging C1 - through R1. When the output of gate 1 is high - pin 3 will charge C1 through R1. And when the output of gate 1 is low - C1 will discharge into pin 3 - through R1. ============ You'll notice an initial delay - while a high pin 3 charges C1. But the instant C1 takes pins 1 & 2 high - pin 3 will go low - and C1 will begin to discharge back into pin 3. And the instant C1 takes pins 1 & 2 low - pin 3 will go high - and begin to charge C1 again. In this way - pins 1 & 2 will keep changing back and forth between high and low - thereby hovering around the switching voltage. ============ The values of C1 and R1 were chosen to produce a relatively slow rate of charge and discharge. For the 4001 and the 4011 - the difference between a high input pin and a low input pin is very small - perhaps less than 0v1. The 100uF capacitor and 1meg resistor should give your digital meter time to react to this small voltage change. ============ In the case of the 4093 - matters are somewhat different. The Cmos 4093 is the Schmidt-Trigger version of the 4011. While its logic is identical to that of the 4011 - its switching characteristics are quite different. ============ In the case of the 4011 - the difference between a high and a low input pin - is only a small fraction of a volt. In the case of the 4093 - the difference between a high and a low input pin - can be several volts. ============ In effect - the 4093 has two separate switching voltages - an upper and a lower. It waits for the input voltage to rise well above half the supply voltage - before it takes the output pin low. And it waits for the input voltage to fall well below half the supply voltage - before it takes the output pin high again. ============ The gap between the upper and the lower switching voltages - is called the hysteresis voltage. And the size of the hysteresis voltage - varies with the supply voltage. ============ With a 12v supply - the hysteresis voltage of a 4093 gate is typically about 2.5v For example - the voltage on the input pins may have to rise above say 7v3 - before the output pin will go low. And it may have to fall below say 4v7 - before the output pin will go high again. The value of the hysteresis voltage is simply the difference between the two meter readings. ============

Circuit No.1 - Inverters

In the case of inverters - the state of the output pin is always opposite to that of the input pins.

That's why they're called inverters.

When the input pins are high - the output pin will be low.

And when the input pins are low - the output pin will be high.

                 ============

An input pin is high when it's more than half of the supply voltage - and low when it's less than half of the supply voltage. 

Roughly speaking - with the 12v supply -anything over 7v is a high - and anything below 5v is a low. 

Somewhere between 5v and 7v is the crossover - or switching point - of the inverter. 

Depending on whether the voltage is rising or falling - when it crosses this point - the output pin will switch from high to low - or vice-versa. 

                 ============

The circuit on the left should work for most common inverters.

But the connections in the drawing are for the Quad Two-Input variety.

These are the 4001 - the 4011 - and the 4093.

                 ============

You only need to measure the voltage on one of the gates.

All four gates were manufactured at the same time - using the same materials.

So they should all have the same characteristics.

                 ============

The circuit works by repeatedly charging and discharging C1 - through R1.

When the output of gate 1 is high - pin 3 will charge C1 through R1.

And when the output of gate 1 is low - C1 will discharge into pin 3 - through R1.

                 ============

You'll notice an initial delay - while a high pin 3 charges C1.

But the instant C1 takes pins 1 & 2 high - pin 3 will go low - and C1 will begin to discharge back into pin 3.

And the instant C1 takes pins 1 & 2 low - pin 3 will go high - and begin to charge C1 again.

In this way - pins 1 & 2 will keep changing back and forth between high and low - thereby hovering around the switching voltage.

                 ============

The values of C1 and R1 were chosen to produce a relatively slow rate of charge and discharge.

For the 4001 and the 4011 - the difference between a high input pin and a low input pin is very small - perhaps less than 0v1.

The 100uF capacitor and 1meg resistor should give your digital meter time to react to this small voltage change.

                 ============

In the case of the 4093 - matters are somewhat different.

The Cmos 4093 is the Schmidt-Trigger version of the 4011. 

While its logic is identical to that of the 4011 - its switching characteristics are quite different.

                 ============

In the case of the 4011 - the difference between a high and a low input pin - is only a small fraction of a volt.

In the case of the 4093 - the difference between a high and a low input pin - can be several volts.

                 ============

In effect - the 4093 has two separate switching voltages - an upper and a lower.

It waits for the input voltage to rise well above half the supply voltage - before it takes the output pin low.

And it waits for the input voltage to fall well below half the supply voltage - before it takes the output pin high again.

                 ============

The gap between the upper and the lower switching voltages - is called the hysteresis voltage.

And the size of the hysteresis voltage - varies with the supply voltage. 

                 ============

With a 12v supply - the hysteresis voltage of a 4093 gate is typically about 2.5v

For example - the voltage on the input pins may have to rise above say 7v3 - before the output pin will go low.

And it may have to fall below say 4v7 - before the output pin will go high again.

The value of the hysteresis voltage is simply the difference between the two meter readings.

                 ============

Circuit No.2 - Non-Inverters
	In the case of non-inverters - the state of the output pin is always the same as the state of the input pins. When the input pins are high - the output pin will be high. And when the input pins are low - the output pin will be low. ============ An input pin is high when it's more than half of the supply voltage - and low when it's less than half of the supply voltage. Roughly speaking - with the 12v supply -anything over 7v is a high - and anything below 5v is a low. Somewhere between 5v and 7v is the crossover - or switching point - of the gate. Depending on whether the voltage is rising or falling - when it crosses this point - the output pin will switch from high to low - or vice-versa. ============ The circuit on the left should work for most of the common Cmos non-inverting gates. But the connections in the drawing are for the Quad Two-Input variety. These are the 4071 and the 4081. ============ You only need to measure the voltage on one of the gates. All four gates were manufactured at the same time - using the same materials. So they should all have the same characteristics. ============ When power is first applied - C1 is in a discharged state. So pins 5 & 6 are low. This means that pin 4 is also low. ============ Because pin 4 is low - it cannot supply current to the base of Q1. So the transistor is switched off. ============ Because Q1 is switched off - pins 1 & 2 are held high by R3. Because pins 1 & 2 are high - pin 3 will be high And pin 3 will begin to charge C1 - through R1. ============ Eventually - the voltage across C1 will take pins 5 & 6 high. So pin 4 will go high - and current from pin 4 will switch the transistor on. ============ When the transistor switches on - it connects pins 1 & 2 to ground. That is - it takes pins 1 & 2 low - causing pin 3 to go low. When pin 3 goes low - it starts to discharge C1 - through R1. ============ As soon as the voltage on C1 starts to fall - it takes pins 5 & 6 low. So pin 4 goes low - and switches the transistor off. R3 takes pins 1 & 2 high - and pin 3 starts to charge C1 again. ============ As soon as the voltage on C1 starts to rise - it takes pins 5 & 6 high. So pin 4 goes high - and switches the transistor on. The transistor takes pins 1 & 2 low - and pin 3 starts to discharge C1 again. ============ The circuit works by repeatedly charging and discharging C1 - through R1. The instant pins 5 & 6 go high - C2 will start to discharge into pin 3. And - the instant pins 5 & 6 go low - C2 will start to charge from pin 3. In this way - pins 5 & 6 will keep changing back and forth between high and low - thereby hovering around the switching voltage. ============ The values of C1 and R1 were chosen to produce a relatively slow rate of charge and discharge. The difference between a high input pin and a low input pin is very small - perhaps less than 0v1. The 100uF capacitor and 1meg resistor should give your digital meter time to react to this small voltage change. ============

Circuit No.2 - Non-Inverters

In the case of non-inverters - the state of the output pin is always the same as the state of the input pins.

When the input pins are high - the output pin will be high.

And when the input pins are low - the output pin will be low.

                 ============

An input pin is high when it's more than half of the supply voltage - and low when it's less than half of the supply voltage. 

Roughly speaking - with the 12v supply -anything over 7v is a high - and anything below 5v is a low. 

Somewhere between 5v and 7v is the crossover - or switching point - of the gate. 

Depending on whether the voltage is rising or falling - when it crosses this point - the output pin will switch from high to low - or vice-versa. 

                 ============

The circuit on the left should work for most of the common Cmos non-inverting gates.

But the connections in the drawing are for the Quad Two-Input variety.

These are the 4071 and the 4081.

                 ============

You only need to measure the voltage on one of the gates.

All four gates were manufactured at the same time - using the same materials.

So they should all have the same characteristics.

                 ============

When power is first applied - C1 is in a discharged state.

So pins 5 & 6 are low.

This means that pin 4 is also low.

                 ============

Because pin 4 is low - it cannot supply current to the base of Q1.

So the transistor is switched off.

                 ============

Because Q1 is switched off - pins 1 & 2 are held high by R3.

Because pins 1 & 2 are high - pin 3 will be high

And pin 3 will begin to charge C1 - through R1.

                 ============

Eventually - the voltage across C1 will take pins 5 & 6 high.

So pin 4 will go high - and current from pin 4 will switch the transistor on.

                 ============

When the transistor switches on - it connects pins 1 & 2 to ground.

That is - it takes pins 1 & 2 low - causing pin 3 to go low.

When pin 3 goes low - it starts to discharge C1 - through R1.

                 ============

As soon as the voltage on C1 starts to fall - it takes pins 5 & 6 low.

So pin 4 goes low - and switches the transistor off.

R3 takes pins 1 & 2 high - and pin 3 starts to charge C1 again.

                 ============

As soon as the voltage on C1 starts to rise - it takes pins 5 & 6 high.

So pin 4 goes high - and switches the transistor on.

The transistor takes pins 1 & 2 low - and pin 3 starts to discharge C1 again.

                 ============

The circuit works by repeatedly charging and discharging C1 - through R1.

The instant pins 5 & 6 go high - C2 will start to discharge into pin 3.

And - the instant pins 5 & 6 go low - C2 will start to charge from pin 3.

In this way - pins 5 & 6 will keep changing back and forth between high and low - thereby hovering around the switching voltage.

                 ============

The values of C1 and R1 were chosen to produce a relatively slow rate of charge and discharge.

The difference between a high input pin and a low input pin is very small - perhaps less than 0v1.

The 100uF capacitor and 1meg resistor should give your digital meter time to react to this small voltage change.

                 ============

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Measuring Cmos Switching Voltages And Hysteresis

Measuring Cmos Switching Voltages
And Hysteresis