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
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.