AND8054 Datasheet, PDF (17/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
Voltage Limit Circuits
Automatic Gain Control (AGC) circuits and voltage limit
or bounding circuits are used in oscillators to prevent the
operational amplifiers from saturating and to avoid
amplifier slew rate limitations. Bipolar transistors are
inherently slow in coming out of saturation; therefore, a
limit circuit should be used to prevent a frequency error
when using amplifiers such as the BiCMOS MC33501 or
MC33503. FET transistors do not have the slow recovery
time problem coming out of saturation; however, a limit
circuit should also be used with CMOS operational
amplifiers. The gain of the transistors in a CMOS
operational amplifier such as the NCS2001 will change
when the transistors saturate; thus, a limit circuit is
necessary to prevent an oscillation error.
Limit circuits will also decrease the required time for the
oscillation signal to stabilize at start–up. When an
oscillator’s poles are located exactly on the imaginary axis,
the resulting waveform will be a perfect sinusoidal signal.
To ensure oscillation startup the poles are adjusted to lie
slightly in the right half s–plane causing the signal to grow
exponentially until it is limited by some type of
non–linearity, such as the saturation voltage of the amplifier.
AGC Circuits
Automatic Gain Control (AGC) circuits provide a linear
control of the amplifier gain to produce a constant output
voltage regardless of the level of the input signal. AGC
circuits are usually used in applications where the level of
signal distortion needs to be minimized. AGC circuits are
more complex than limit circuits and usually consist of an
operational amplifier and/or FET that are used as a variable
resistor. An example of an AGC circuit is shown in Figure 17.
V_CC
V_IN
+ ^U1
–
R1
V_EE
R2
Q1
AGC
V_OUT
Figure 17. FET AGC Circuit
Limit Circuits
Limit circuits are nonlinear circuits, which clamp the
amplitude to a voltage level that is less than the amplifier
power supply voltage. This clamping function will produce
distortion in the oscillator signal. The selection of the
voltage limit circuits is based on the allowable signal
distortion and the simplicity of the circuit. The distortion
level for most sensor oscillator circuits is relatively
unimportant because only the frequency of the signal is
monitored. Also, limit circuits are preferable to AGC
circuits because they require fewer components. Limit
circuits typically consist of a combination of zener diodes,
diodes, and transistors.
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