Inverter oscillator

[Kevin Weddle]

I'm designing a simple inverter oscillator. There are one or two designs depending on if the logic is bipolar transistor or fet. Is it the bipolar transistor logic which requires a 0v sink current? Does the fet logic only require nothing to output high?

 

[audioguru2]

An old TTL logic device is bipolar. Most TTL inverter oscillators use a crystal, not an RC network.

A Cmos oscillator can use an RC network for a fairly accurate frequency. Ordinary inverters can be used or a single Schmitt-Trigger inverter.

I do not know what you mean when you say "0V sink current" and "nothing to output high".

View attachment 41953

 

[Hero999]

You can make an astable multivibrator with TTL but you need to use lower value resistors and higher value capacitors, as the input impedance is low.

View attachment 41954

 

[Kevin Weddle]

I do not know what you mean when you say "0V sink current" and "nothing to output high".

The difference between the TTL and CMOS inverter oscillator if I recall is the CMOS inverter following the feedback capacitor needed a gate discharge resistor. The TTL circuit used a pullup instead of pull down resistor for this difference. Was I missing the gate discharge resistor?

 

[audioguru2]

The difference between the TTL and CMOS inverter oscillator if I recall is the CMOS inverter following the feedback capacitor needed a gate discharge resistor. The TTL circuit used a pullup instead of pull down resistor for this difference. Was I missing the gate discharge resistor?

There is no "gate discharge resistor". The phase shift of C1 and R2 sets the oscillator frequency and R1 limits the current into the input protection diodes so that the voltage at C1 can swing higher and lower than the supply voltage without changing the frequency.View attachment 41955

 

[Hero999]

Although the oscillator will normally work with two gates, it may fail to oscillate if C is small.

A three gate oscillator is more reliable.
http://www.fairchildsemi.com/an/AN/AN-118.pdf

View attachment 41956

 

[Kevin Weddle]

I finished the circuit. It is uses two inverters, capacitors, and resistors. The resistors pull up and the output is about a cycle every few seconds. There isn't a gate resistor connected to ground.

 

[audioguru2]

I finished the circuit. It is uses two inverters, capacitors, and resistors. The resistors pull up and the output is about a cycle every few seconds. There isn't a gate resistor connected to ground.

Since you did not post a detailed schematic then we do not know what you are talking about. You didn't even say if the inverters are TTL or Cmos, and did not say their part number.

 

[Kevin Weddle]

It's a CMOS inverter oscillator. The voltage will double, then the inverter will fail.

 

[audioguru2]

It's a CMOS inverter oscillator. The voltage will double, then the inverter will fail.

What voltage will double? The supply voltage? How can it double?
Where is your schematic and parts list?

 

[Kevin Weddle]

It's the charge and discharge on the capacitor. A discharged capacitor doesn't have to discharge, a charged capacitor has to discharge.

 

[audioguru2]

It's the charge and discharge on the capacitor. A discharged capacitor doesn't have to discharge, a charged capacitor has to discharge.

The capacitor charges and discharges with alternating current that goes positive then it goes negative.
Here is a sketch of the waveform at the capacitor:View attachment 41957

 

[Kevin Weddle]

The original circuit. The voltmeter shows a positive voltage on the positive cycle always 300mV.

View attachment 41960

 

[Kevin Weddle]

At 1cycle/3s the slew rate is probably too low to double the voltage. The 5kHz variation stopped working, probably it's a bad inverter. Don't have another one.

 

[audioguru2]

Kevin, your .68K capacitors are not 680uF. Instead they are 0.68uF which is 680nF.
5M x 0.68uF= 3.4 seconds. The K indicates a tolerance of 10%.

Your circuit is almost the same as my Standard Cmos Inverters Oscillator when I turn it around. Except it has an extra resistor and extra capacitor, its lower resistor is connected to the wrong place and it is missing a resistor that prevents the input protection diodes on the Cmos inverter from conducting and messing up the timing. Try the Standard circuit with a 330nF capacitor for a 3 seconds cycle.

The slew rate for a Cmos inverter is very fast.

Your circuit CANNOT double the voltage on the capacitor because the input protection diodes conduct and clamp the voltage to a diode voltage drop above the supply voltage and a diode voltage drop below ground. Then the timing is messed up.  

View attachment 41962