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Such a functional unit as an input amplifier-shaper is a necessary component of a digital scale-frequency meter for signals whose level does not exceed tens of millivolts. The signal applied to the input of such an amplifier is amplified to a level sufficient for the stable operation of digital pulse counters. The amplifier must not have a shunt effect on the circuit to which the digital scale-frequency meter is connected, and therefore its input resistance must be high and its input capacitance low. In addition, the signal with digital levels generated at the output of the amplifier should not contain parasitic pulses that can introduce an error into the frequency measurement by electronic counting.

After a number of experiments, including unsuccessful ones, the author proposes the schematic shown in Fig.1. The actual counting pulses at the output are generated by the Schmitt trigger D1:1 (one of the six elements of the popular SN74HC14N chip was used). At an ambient temperature of +25°C and a supply voltage of 5V, the typical levels of the upper and lower thresholds for such a Schmitt trigger are slightly more than +2,5V and slightly less than +1,6V, respectively. The formation of counting pulses is explained by the diagram in Fig.2.

The blue sinusoid in Fig. 2 is the analog signal at the input of the Schmitt trigger, the red color shows the digital rectangular signal at its output. From the low state at the output, the trigger switches to the high state at the moment when the input signal level drops below the threshold indicated on the diagram by the green horizontal line – this is the lower threshold for the Schmitt trigger. The next switching of the trigger, but already back – from the high state to low – occurs only when the signal level at the input exceeds the upper threshold, indicated by a purple horizontal line. Then the input signal level should cross the lower threshold again, and so on. Therefore, for reliable switching of the Schmitt trigger, the amplitude of the variable component of the signal at its input must be greater than its upper and lower thresholds are separated from each other, provided that the level of the DC component of the signal at the trigger input is located between these thresholds in the middle. At the same time, various interferences with a smaller amplitude cannot cause faulty switching of the Schmitt trigger and affect the result of frequency measurement.

This operation of the Schmitt trigger D1: 1 provides an input signal amplifier on transistors VT1, VT2 and VT3. The DC-voltage on the collector of transistor VT3 connected to the trigger input is maintained within the limits between the levels of the upper and lower thresholds of the Schmitt trigger during fluctuations over a wide range of ambient temperature (see the diagram in Fig.3) and the supply voltage +V_c (see the diagram in Fig.4). This became possible due to the negative DC feedback, which covers the amplifier by supplying part of the DC voltage from the output of the circuit to the current-setting circuit of a stable current source made on a field-effect transistor VT1. The same transistor works as the first stage of amplifying the input AC signal. On bipolar transistors VT2 and VT3, connected according to the cascode circuit, the second amplification stage is made.

To determine the sensitivity of the amplifier-shaper assembled according to the circuit in Fig.1, a sinusoidal signal with a frequency of 1 MHz was applied to its input from the high-frequency generator, by gradually increasing the level of which the threshold was determined, where reliable formation of counting pulses at the output was already ensured. The frequency response of the shaping amplifier was determined by successively repeating this operation with an increase in the frequency of the input signal: at frequencies up to 4 MHz, the sensitivity was no worse than 10mV, and at a frequency of 32 MHz it was about 30mV. The decrease in the sensitivity of the amplifier-shaper with increasing frequency of the input signal is characterized by the diagram in Fig.5. The good frequency properties of the amplifier are due to the use of high-frequency transistors and the cascode construction of the second amplification stage.

Equally important is the stable operation of the shaping amplifier even at a high input signal level, when the amplifier already operates as a limiter, that is, outside the relatively linear region of its transfer characteristic. Periodically repeating transients in this case can generate at the output of the amplifier – the input of the Schmitt trigger – unnecessary voltage drops and, as a result, false pulses at the output. Fig.6 shows an oscillogram with a voltage waveform at the collector of transistor VT3 when a signal with a frequency of 1 MHz and a level of more than 70mV is applied to the input of the amplifier. From the rounded corners of the “trapezoids” it is clearly seen that the amplifier based on transistors VT1, VT2 and VT3 “softly” enters the limiting mode and also “softly” comes out of it, that is, without any signs of transients (bursts, damped oscillations, etc.). It is clear that with such an operation of the amplifier, there can be no talk of any false pulses at the output of the Schmitt trigger.

To ensure that the voltage at the input of the Schmitt trigger D1: 1 does not go beyond the permissible limits, the upper level of the voltage limit on the collector of the transistor VT3 should be set slightly lower than the supply voltage of the trigger microcircuit. In the circuit in Fig.1, this level is set by a divider composed by resistors R7 and R6, the voltage from which is supplied to the base of the transistor VT3. The resistor R7 of the divider is connected to the +V_d digital chips power bus, including the D1 chip, and therefore, as can be seen in the oscillogram in Fig.6, the voltage on the collector of the transistor VT3 does not leave the range from 0V to +5V for V_d = 5V. With an other value of V_d, the upper level of the voltage limit on the collector of the transistor VT3 will also be other.

Setting up the amplifier-shaper comes down to setting the voltage on the collector of transistor VT3 by resistor R3 so that its value lies approximately in the middle between the upper and lower thresholds of the Schmitt trigger D1:1. For the SN74HC14N chip, with a supply voltage of V_d = 5V, this is about 2.0 … 2.1V. Another way to set the required voltage is as follows. A sinusoidal signal with a frequency of 0.2 … 1MHz and a level one and a half to two times higher than the sensitivity threshold, say 20mV, should be applied to the input of the amplifier-shaper from a high-frequency generator, and adjust by R3 resistor the shape of a rectangular digital signal close to “meander”, that is, with equal duration of low and high state of it.

The current consumed from the +V_c power supply by a correctly assembled and adjusted amplifier-shaper does not exceed 5mA.

The maximum allowable input signal level is determined by the gate-source cutoff voltage of the field-effect transistor VT1. It should be noted that with a higher input signal level, an amplifier may not be needed – with the correct connection, one Schmitt trigger will suffice.

The article presents the effective values of signal voltages.

Copyright © Sergii Zadorozhnyi, 2008