Op-amp Oscillator Circuit Design (10-50 MHz)

In summary, the conversation discusses the use of an op-amp to create an oscillator at 10MHz. The first question asks for an affordable op-amp for this operation, while the second question asks about the likelihood of a beginner being able to design a zero phase shift feedback filter for a non-inverting op-amp circuit. The third question asks for advice on the feedback circuit for an inverting op-amp circuit. The conversation also delves into the use of an unbuffered CMOS inverter gate and a crystal for the oscillator and the various applications for the oscillator. The summary concludes with a
  • #1
tadeh89
8
8
TL;DR Summary
Hi everyone,

I have a good understanding of how an oscillator circuit is supposed to work, meaning zero phase shift and gain bigger than unity; however, I have not been able to make one work in practice. I am trying to utilize an LM741 op-amp. I have tried a colpitts circuit, but I will use any topology at this point just to see it work at this high of a frequency, 10-50MHz.
Questions:

1. Is LM741 capable of oscillating at 10MHz? If not, could you suggest me an affordable op-amp for this operation?
2. How likely am I, as a beginner to be able to design a zero phase shift feedback filter to use with a non-inverting op-amp circuit to create an oscillator?
3. If an inverting op-amp circuit is the way to go, how should I go about the feedback circuit? I have been trying to make a feedback circuit with low loss in the pass-band (-6dBish), and 180 degrees phase shift, but I can't get both of them at the same time. At best, I was able to get -13dB loss and 180 degrees phase shift, but I am unsure if my op-amp gain can compensate for 13dB plus a little.

Please take a look at my simulation for the filter/feedback portion in ADS. (images forum1 and forum2 belong to the same simulation, forum 3 is independant)

Any help/input will be appreciated.

Thanks all!
 

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  • #2
Yeah, 10-50MHz is way too fast for a typical opamp oscillator, IMO. Opamps are more for audio frequency oscillators.

What accuracy do you need from this oscillator? Can you just use an unbuffered CMOS inverter gate and a crystal in either parallel or series oscillation mode? What does this oscillator feed?
 
  • #3
berkeman said:
Yeah, 10-50MHz is way too fast for a typical opamp oscillator, IMO. Opamps are more for audio frequency oscillators.

What accuracy do you need from this oscillator? Can you just use an unbuffered CMOS inverter gate and a crystal in either parallel or series oscillation mode? What does this oscillator feed?

Thanks you for responding.

It does not need to be very accurate, at least for now, as it is my weekend project., but what would be the next step sort of design for a more accurate frequency? Your comment lead me to the datasheet for 74LVU04.
1587147478820.png


Is this what you have in mind? If yes, then is this considered series? And if I wanted to try the crystal in parallel, I would have to have two inductors in series, and the crystal in shunt? I apologize if I don't make any sense.

I will use this oscillator for different purposes, such as referencing a PLL phase detector, or to feed a frequency multiplier. I am a newbie in the RF field. I think I have a very basic understanding about all of the RF blocks, which really makes me want to design a cheap version of each block at home to better my understanding.

Thanks for your help
 

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  • #4
tadeh89 said:
Your comment lead me to the datasheet for 74LVU04.
1587147478820-png.png


Is this what you have in mind?
Yes, that is the classic parallel mode circuit for a crystal oscillator. The crystal you choose will have a parallel resonant capacitance value associated with it in the datasheet (like 30pF for example), and you will choose the two caps shown in that diagram so that their series combination in parallel with the gate capacitance is close to that rated capacitance in the datasheet. R2 is typically a couple hundred Ohms, and R1 is 1Meg Ohm or similar. Keep the output loading on the U04 low, like have it feed only one other IC or use a buffer gate if you need to fan out the clock signal.

tadeh89 said:
I will use this oscillator for different purposes, such as referencing a PLL phase detector, or to feed a frequency multiplier.
For those applications you want an accurate time base, so using this crystal oscillator circuit is a good match.

BTW, I'm not familiar with the circuit that you show at the bottom of your post with the inductor across the gate.
 

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Last edited:
  • #5
I will get on it and I will update with the results. Thanks again for your help!
 
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  • #7
tadeh89 said:
2. How likely am I, as a beginner to be able to design a zero phase shift feedback filter to use with a non-inverting op-amp circuit to create an oscillator?
3. If an inverting op-amp circuit is the way to go, how should I go about the feedback circuit?

* For a non-inverting amplifier the feedback network is, in most cases, a bandpass (RC-CR or RLC) which has zero phase shift at the mid-frequency (Example: WIEN oscillator)
* For an inverting amplifier , in most cases we are using a third-order lowpass or highpass (Example: Phase-shift oscillator) or - as a very useful modification - a second-order lowpass with an inverting MILLER integrator (instead of an amplifier)
 

1. What is an op-amp oscillator circuit?

An op-amp oscillator circuit is a type of electronic circuit that uses an operational amplifier (op-amp) to generate a periodic output signal without the need for an external input signal. It is commonly used in electronic devices such as radios, televisions, and computers.

2. How does an op-amp oscillator circuit work?

An op-amp oscillator circuit works by using positive feedback to create a continuous oscillating output signal. The op-amp is configured in a feedback loop with a frequency-determining network, such as a resistor-capacitor (RC) network, that sets the oscillation frequency. The output of the op-amp is then fed back to its input, causing the signal to continually oscillate.

3. What is the frequency range of an op-amp oscillator circuit?

The frequency range of an op-amp oscillator circuit can vary depending on the specific design, but it typically ranges from 10 MHz to 50 MHz. This range can be adjusted by changing the values of the frequency-determining components in the circuit.

4. What are the key factors to consider when designing an op-amp oscillator circuit?

There are several key factors to consider when designing an op-amp oscillator circuit, including the desired frequency range, stability, and distortion. The choice of op-amp, frequency-determining components, and feedback network all play a role in the overall performance of the circuit.

5. What are some common applications of op-amp oscillator circuits?

Op-amp oscillator circuits have a wide range of applications in electronics, including signal generation, frequency synthesis, and clock circuits. They are also commonly used in audio and radio frequency (RF) circuits, as well as in test and measurement equipment.

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