#332 Oscillators/RCPhaseShiftBJT

Testing an RC phase-shift oscillator using a BJT amplifier.

Build

Notes

Given an inverting amplifier (180˚ phase shift), with output fed back into a network that adds another 180˚ phase shift, and provided the amplifier gain is sufficient to provide overall positive feedback gain of 1 … then we have the basis of an oscillotor.

The RC phase-shift oscillator with BJT amplifier is one of the simplest realisations of this, requiring no special components:

an NPN BJT in a common-emitter configuration is the inverting amplifier
a cascade of RC high-pass filters provide the additional 180˚ phase shift
the amplifier provides enough gain to overcome losses in the RC network

The basic BJT/RC phase shift oscillator is good for low(er) frequency sine wave generation. Main drawbacks:

performance is very component-specific. Actual frequency may be far of theoretical.
variable frequency control is non-trivial enhancement

RC High-pass Filter Phase Shift

Capacitive Reactance: Xc = 1/2πfC
Impedance: Z = √(R² + Xc²)
phase angle: ø = arctan(Xc/R)

A single filter output leads by at most approaching 90˚, therefore three filters in series are required to achieve ≥ 180˚. More stages could be used, but this adds more error and variation.

The frequency of the filter network is predicted based on the -3dB cut-off: f = 1/(2πRC√(2N)), where N is the number of filter stages.

The total phase shift is then: ø = N.arctan(1/(2πfRC))

So in the basic R=10kΩ, C=10nF configuration I’m using here, we’d expect frequency of 650Hz with a phase shift for each stage of 67˚, for a total phase shift of 201˚ - safely over the 180˚ minimum required.

Biasing the BJT

The circuit I’m using is probably the canonical demonstration. The BJT is in a Class A common-emitter amplifier configuration with:

voltage divider bias
emitter resistor Re. Higher Re stabilises emitter current, making it more independent of the transisitor ß/hFE.
a bypass capacitor in parallel with Re improves AC gain. Set Xc≤0.10RE for lowest frequency.

Results on a Breadboard

RCPhaseShiftBJT_bb_build

Notes and modifications I’ve made after the initial schematic design:

add C5 (10nF) to smooth the power supply; this helped stabilise the oscillator significantly
had to play around with the value of Re to ensure the transistor stayed in the linear region. Settled on 360Ω.
for this biasing arrangement, requires a minimum of 8.5V supply for the oscillator to get started

R=10kΩ, C=10nF

Produced a 746 Hz oscillation in practice, 14% above predicted. Not very surprised - especially on a breadboard. The phase shift per pole of the filter should be around 64˚

With a 9V supply, it produces a very nice sine wave at 3.66V peak-to-peak.

scope_bb_10nF_td

scope_bb_10nF_fft

R=10kΩ, C=100nF

I randomly switch the capacitors to 100nF and it slows things down to a very nice sine wave at 103Hz.

But this is curious: the phase shift per pole would presumably be 57˚, and one would think the oscillation couldn’t be maintained.

Since I know these ceramic 100nF caps tend to err significantly low (sometimes down to ~90nF), I suspect this is only working for me accidentally doe to parts tolerance.

scope_bb_100nF_td