Function Generator Circuit Concepts, Part 2: Current-Source Function Generators (FGs)

The first-generation integrator-hysteresis-switch FGs were replaced by a second-generation design scheme using current sources. The integrator-based FG loop can be improved by driving the integrating op-amp with a current and omitting the timing resistors. Furthermore, if the hysteresis switch selects between two bipolar current sources as input, then the sources can be adjusted for accurate current amplitudes and the amplitudes are less dependent on the switching dynamics. A third and important change is to do away with the op-amp integrator, for a current source driving a timing capacitor will produce a triangle-wave just as well, and faster. The result of these changes is the current-source FG. The basic scheme is shown below.

A current source of positive or negative polarity is switched (by the DPDT switch) into timing capacitor, C. The resulting ramp is buffered by the ×1 voltage amplifier to output the triangle-wave. It is input to the hysteresis switch which outputs the square-wave. It also selects the polarity of timing current.

A circuit implementation based on this general scheme (and was used in the HP8082A) is shown below.

The hysteresis switch consists of Q1, Q2, and Q4. The feedback to Q2 from Q4 makes the circuit bistable, and Q3 buffers the square-wave to set the voltage level at the input of the diode bridge. When the level is high, D3 conducts, D1 and D4 turn off, and the current from the high-side source flows into C through D2. This generator loop is optimized in design more for speed than precision. This is evident in the relatively low input resistance of the hysteresis switch BJT, Q1. The hysteretic comparator is optimized for speed and could be implemented using an ECL gate. The comparator threshold voltages are determined by R0, RL1, R1, and R2 along with BJT junction drops – not the most precise determinant of levels. Consequently, the frequency range for a given C will not be as many decades as for a precision FG, but this FG will have high frequency generation capability of 100 MHz or more.

A variation on the current-source FG which simplifies the switching from a DPDT switch to a SPDT switch is shown below.

One of the sources is not switched, and to offset its current, the other source is made 2xI in magnitude. The two sources must still be balanced in ratio for waveform symmetry but the switching is simpler.

Another switch variation is to make it differential and use both polarities of output from the hysteretic comparator, as shown below.

The current switches are BJT diff-amp pairs that can switch quickly while the timing current is set by the sources that drive their emitters. The other output from each of the diff-amp pairs drives the other input of the other diff-amp, thus speeding switching.

Although arbitrary waveform generators and digital synthesizers are replacing some analog FGs, the circuits found in them are worth remembering and using. Because analog is inherently faster than digital, analog function generation will remain a contender with digital by providing greater time resolution and for clean implementations, even amplitude resolution. Past efforts of Intersil, then Exar, then (much later) Maxim to integrate the integrated-circuit FG are noteworthy and hopefully not the final efforts. The optimum FG might be a μC-based analog FG such as an earlier example, the HP3314A. By combining the best functional features of digital and analog, FGs should continue to be attractive instruments to have on an electronics workbench. By knowing FG circuits, the engineer is better equipped with circuit possibilities in designs requiring such functions.

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