Probe for checking the suitability of operational amplifiers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Operational amplifiers (op amps) are widely used by radio amateurs in the design of various radio devices. Moreover, in the context of the growing high cost of radioelements and their shortage, it is sometimes necessary to use microcircuits that have already been used earlier in the work. To be sure of the suitability of such an op-amp, it should be checked, for example, using a probe described in [L].
However, practical tests of this device have shown that when checking some series of op-amps (such as KR544UD1B, K153UD2), the probe always signals a malfunction of these microcircuits, regardless of their state.
After analyzing the operation of the device and operating modes of the op-amp, I managed to find out the reason for such selective "behavior" of the probe and, by eliminating it, significantly expand the range of tested amplifiers.
Schematic diagram of the upgraded probe is shown in Fig.1. In practice, it is not much different from its predecessor: diodes VD1-VD2 are included in the base circuit of the transistor VT4, the values of some resistors are changed.
Fig.1 (click to enlarge)
The op amp under test is connected to the sockets of the X1 connector (as an example, the connection of the op amp K140UD2 is shown). Such an inclusion forms a relaxation generator that generates rectangular pulses (meander) with a frequency of 1 ... 2 Hz. The supply voltage is supplied to the generator from the parametric stabilizer R1VD1.
If the op-amp turns out to be suitable, the generator will start working, and the HL1 LED will flash in time with the frequency of the generated pulses.
If the tested op-amp turns out to be unusable, the generator will not work, and the LED, depending on the cause of the amplifier’s malfunction, will either burn continuously or not flash at all.
What is the reason that when testing suitable op-amps of the KR544UD1B, K153UD2 series with a probe [L], the HL1 LED signaled a malfunction of the amplifiers?
When taking the oscillogram at point "a", it can be seen that the minimum voltage (U2, Fig. 2, a) of the generated pulses is too high in absolute value to close the transistor of the npn structure (depending on the op-amp series, this voltage can reach a value of 2 V): U2 > U1, where U1 is the voltage threshold at which the emitter junction of the transistor opens. Therefore, despite the fact that the generator is working (because the microcircuit is working), the transistor VT1 is constantly open, and the HL1 LED is on, indicating that the microcircuit is unusable.
Ris.2
To reduce the voltage at point "a", diodes VD1-VD2 are included in the base circuit of the transistor VT4. Now the oscillogram at this point has the form shown in Fig. 2b: the minimum voltage of the generated pulses is less than the threshold value of the emitter junction of the transistor. The transistor will open and close, and the LED will flash at the frequency of the generated pulses.
In addition to those indicated in the diagram, transistors KT312A-KT312V, KT315A, KT315V-KT315I, KT503A-KT503E, diodes KD521A-KD521G, KD103A, KD103B, zener diode D814 G can be used in the probe. .
The details of the device are placed on a printed circuit board (Fig. 3), made of one-sided foil fiberglass with a thickness of 1 ... 1.5 mm.
Ris.3
A properly assembled probe does not require adjustment.
Using a probe, you can check almost all the op-amps most used in practice, except for those whose output impedance is comparable to or exceeds the resistance of the resistor R7, for example, micropower op-amps K140UD12, K153UD4.
References:
- Kozlov F., Prilepko A. "Cube" for checking OS. - Radio. 1986, no. 11, p. 59.
Author: S. Kapustin
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