Wanted: Stable oscillator

When dealing with high speed amplifiers, it’s common to have unwanted oscillations because of parasitic or loop gain issues.  It’s possible to predict the frequency range of these oscillations, but not to target a specific frequency.  So how do we create an oscillator with a specific frequency?

There are various approaches.  Many oscillator circuits are based on transistors, but some are adapted to use operational amplifiers.  Here we will make use of an Operational Transconductance Amplifier (OTA) to create a linear oscillator.  The transconductance is the conversion of voltage into current expressed in mA/V or S (Siemens). More information on OTA can be found in the OPA861 product datasheet or in an application note I wrote titled “Demystifying the Operational Transconductance Amplifier.” 

One simple way to consider an OTA is to look at it as a self-biased bidirectional transistor with three terminals: a B-input, an E-input/output and a C-output.  The nomenclature used here emphasizes the resemblance to a transistor.  The B input has the same function as the base of a bipolar transistor, E is the emitter and C is the collector.  The E-input/output is used as either an input or an output depending on the circuit configuration.

Consequently, the B-input is high impedance while the E-input is low impedance with an output impedance of  with gm the transconductance gain in mA/V, and finally the C-output is high impedance.

Figure 1: Parallel LC oscillator

The oscillating frequency is set by   .

RC will set the Q factor, or how wide the spread is around the resonant frequency, while RE will set the gain.  Note that the gain resistor is the sum of the internal impedance of the E-input with the external gain resistance.

In the same manner, don’t forget to take into consideration the parasitic capacitance at the C-output, the B-input and the buffer input nodes when calculating the resonance frequency and to select Cosc as to be the dominant term.

The circuit developed here was implemented using the OPA860 which combines both a high speed OTA and a closed-loop buffer.  To achieve the results shown in figure 2 below, we selected the following components:

RC = 100W (5%)

RE = 24W (5%)

Losc = 12nH (5%)

Cosc = 1nF (X7R ceramic = ±15%)

Due to component tolerances, we expect the oscillation to be between ~41.8MHz and ~51.6MHz.  We measure 43.1MHz for room temperature in accordance with component tolerances.

Figure 2: Resonance frequency variation over temperature

For the plot above, the entire PCB was inserted in the oven, drifting all components together.  The overall center frequency variation is coming from the independent LC elements.  The OTA transconductance gain will vary as well, but as it varies with temperature, the gain will change.  If the gain becomes insufficient, the oscillation will stop.

Improvement would have to be made to minimize the resonant frequency temperature dependency, possibly using calibration. This circuit, if used at room temperature, can be used to measure the capacitance or inductance variation of a system by monitoring the oscillation.  As the capacitance or the inductance of the system varies, the resonant frequency will change providing a relative measurement of the varying element.

Check out my blog post titled “High-gain, high-bandwidth: why is this circuit oscillating” if you’re looking for more info on what to do if you have an oscillation in your design.

And, if you’d like even more reading material, I encourage you to check out my other posts as well.

High-gain, high-bandwidth… how can I get it all?

High-gain, high-bandwidth....putting it all together

Correcting DC errors in high-speed amplifier circuits

Reducing amplifier power consumption for SAR ADC drivers

Current feedback amplifier...how do I make it work for me?

This amplifier doesn't exist...now what!?

This amplifier doesn't exist...now what?! - Part 2

Anonymous
  • Hello, I go back to before transistors were invented. I still have a few CK722's from the old days. (Never mind the 6SN7GT's). I found that if I wanted a circuit to oscillate, I should write "Amplifier" in the title block. Likewise, if I want a stable amplifier, write "Oscillator" in the title block. It usually works.

     Seriously,  figure out how much gain you need to start the oscillator, then use an unbypassed emitter resistor to set the gain.  In the CK722 days, use the gain bandwidth product to calculate the gain needed.   Keep your circuit small and properly grounded.

     I got a circuit back from layout one time. I specified that the bypass caps be next to the IC. They were, but the traces went around Robin Hood's barn to get to the chip. I don't think I can describe my mental state when I looked at the boards...

     Best Regards

     Bill Grenoble

  • Hi Xavier,

    I looked at the datasheet for the OPA861 device and it is easy to see how this device could oscillate. Negative resistance could easily develop. Particularly if the emitter is raised above ground with an emitter resistor. The device could oscillate as a common base or common emitter type oscillator.

    Kind regards,

    Jim

  • Hi Xavier,

    I don’t fully understand the basic nature of transconductance amplifiers?

    I do understand Op-Amps, Instrumentation Amps and differential Amps from a user’s standpoint anyways “most of the time”. I.e. an instrumentation mode Amp is used when high common mode rejection is needed for a balanced-differential input.

    Although current amps may have some differences? The few times I’ve used them I treated the device the same as I would an Op-Amp, but at a higher frequency with reasonable gain and reasionable low resistors to drop the noise. I.e.: say Av = 6 to avoid peaking.

    If you have any basic tutorial or blogs on transconductance amplifies, kindly let me know if possible?

    Kind regards,

    Jim

  • Hi Jim,

    The OPA861 has a 20mA output current drive capability.  Since the quiescent current is relatively low, the BW is relatively low.

    Yes the transconductance amplifier can be used as a current amplifier.  You have one high impedance input receiving the voltage and converting it to a current proportioanlly to a resistors.

    Note that an OTA is not a current feedback amplifier, but is one of its building block.

    Best regards,

    Xavier

  • Hi Ramus,

    It seems if a transistor could source current into a resonant circuit and the total phase is equal to 2 Pi, then it would be an oscillator. I never thought of these types of devices as sourcing appreciable current (say mA’s). Although transistor oscillators can work with currents as low as a milliamp.

    Thank you,

    Jim

    P.S. Also I don’t know much about   operational transconductance amplifiers. Are these current amps?

    If so I’ve used one it was setup like an Op-Amp as far as gain went but had a higher GBW product