Soft limiter for oscillator circuits uses emitter-degenerated differential pair
Phase-shift network plus a few active components yields low-distortion output.
Herminio MartÃnez and Encarna Garcia, Technical University of Catalonia, Barcelona, Spain; Edited by Brad Thompson and Fran Granville -- EDN, October 12, 2006
Most oscillator circuits include a nonlinear amplitude control that sustains oscillations at a desired amplitude with minimum output distortion. One approach uses the output sinusoid's amplitude to control a circuit element's resistance, such as that of a JFET operating in its triode-characteristics region. Another control method uses a limiter circuit that allows oscillations to grow until their amplitude reaches the limiter's threshold level. When the limiter operates, the output's amplitude remains constant. To minimize nonlinear distortion and output clipping, the limiter should exhibit a "soft" characteristic.
Based on a waveform shaper that imposes a soft limitation or saturation characteristic, the circuit in Figure 1 comprises a simple RC (resistor-capacitor)-ladder phase-shift oscillator and an amplitude-control limiter circuit. R1, R2, and R3 have values of 10 kΩ each, and C1, C2, and C3 have values of 1 nF each. The following equation defines output voltage VOUT's frequency, fO.
The inverting-amplifier block in Figure 1 comprises transistors Q1 and Q2, a differential pair that presents a nonlinear-transfer characteristic, plus an IVC (current-to-voltage converter) based on operational amplifier IC1. For oscillation to occur, the inverting amplifier's gain magnitude must exceed 29. Selection of appropriate values of bias current, IEE; the transistor pair's emitter-degeneration resistances, RE1 and RE2; and RE3 produces the amplifier's nonlinear-transfer characteristic, VOUT versus VIN (Figure 2).
A small input voltage produces a nearly linear-amplifier-transfer characteristic. However, large values of input voltage drive Q1 and Q2 into their nonlinear region, reducing the amplifier's gain and introducing a gradual bend in the transfer characteristic. A current mirror comprising Q3 and Q4 converts the shaping circuit's output to a single-ended current, which operational amplifier IC1 converts to an output voltage. In the prototype circuit, calibration trimmer RE3 has a value of approximately 33 kΩ. Figure 3 shows the oscillator's output voltage for the component values in Figure 1, and Figure 4 shows the sinusoidal output's spectral purity.
The nonlinear amplifier's wave-shaping action occurs independently of frequency, and this circuit offers convenience for use with variable-frequency oscillators. Note that IC1's gain-bandwidth product limits the circuit's performance. To use the limiter portion of the circuit with a noninverting amplifier, such as a Wien-bridge oscillator, apply the signal input voltage to Q2's base, and ground Q1's base.
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I agree with the comments from Stan. You could put medium resolution versions of the graphics on the same web page, so that we can read the articles as a single page, and then click the figure if needed for a more detailed diagram. Please do retain the PDF - that is very valuable when high resolution is needed. Regards, Kedar.
Kedar Godbole - 2006-17-10 07:33:00 PDT -
You do not need to click "Printer-friendly" to access the Figures. Clicking "Figure 1" in the text will load Figure 1 only--much faster. We use PDFs (both for the individual figures and for the complete articles) because no technology we know of provides the ability to zoom in as far as you sometimes need to on complex schematics. Putting a large Figure on the Web page would mean some of our Web pages would have to be several screens wide. In addition, PDF is universal, compatible with our production process, and easy to save and print. Thanks for visiting EDN.com!
Matthew Miller, Executive Editor, Online - 2006-13-10 12:46:00 PDT -
This business of having to click on "printer friendly version" then wait for a pdf file with several articles in it to download, in order to see the illustrations and the text on the same page is disgraceful. Why can't they put the text and illustrations on the web page?
Stan Owens - 2006-13-10 04:31:00 PDT
