The One Transistor Pump
This is both a very simple and useful circuit building block. I learned about when I was an undergraduate but haven't seen it used elsewhere.The input is (ideally) a square wave and the output is a voltage staircase.
Operation
Assume that Vo is 5V and that Vin is a series of square waves, height V; also that the transistor has a high ß and is on the verge of conduction so that:
When Vin presents a negative-going edge to C1 its right-hand plate attempts to drop by V, but the transistor turns on and maintains V1 at 4.3V. The diode is reverse-biased so Vo doesn't change.
When a positive edge appears at the input the transistor is cut off and when the input reaches 0.7V the diode is forward-biased and current flows into C2. Charge sharing occurs between C1 and C2 so Vo changes by:
This incremental increase is maintained for each cycle until Vo approaches Vcc which is the maximum output voltage possible. The ouput thus consists of a series of voltage steps of constant height.
Uses - Frequency to Voltage Converter
A frequency-to-voltage converter (see circuit below) can be produced simply by connecting a resistor across the output. As the ouput voltage rises on each cycle the current through the resistor will rise until the charge supplied by the pump during each cycle equals that flowing through the resistor during the same time.
In the steady state the ouput voltage will not be constant but will have some ripple super-imposed.
k = C5/(C5 + C6), V1 = V - 1.4 and τ is the product of R13 and C6.
Design
The circuit for the complete is converter is shown below; T2 and T3 form a Schmitt trigger. The trigger produces pulses of height independent of the shape or amplitude of the input waveform. T1 is included to increase the input sensitivity and to shift the dc operating point to an appropriate one for the Schmitt trigger input. T4 is an emitter follower to provide a low impedance drive for the pump.For correct operation:
where Tp is the period of the highest operating frequency (which is 1 µs for a maximum input frequency of 1MHz)
The output impedance of the emitter follower (in this case) is approximately 6Ω; thus: C5 « 10-6/6; i,e, C5 « 166nF,
Now, referring to the waveform of Vo against time:
Vomax = Vomin + k.V1
Vomin = k.V1/(e-T/τ - 1)
Assume T « τ, so, using the approximation of ex = 1 - x (for x « 1):
Vomin = (C5/(C5 + C6)).V1.C6.R13.f
If C5 « C6, then:
The slope of the voltage-frequency coefficient is given by:
If for an input frequency f = 500 kHz, we require Vo = 2.5 V then for: V1 = (7.5 -1.4) Volts (i,e, the total swing at the emitter of T4 minus two forward bias diode drops) and ensuring that: C6 > 20.C5 (to satisfy the inequality above), we obtain:
= 3.9 kΩ (preferred)
The time constant C6.R13 must be greater than the longest time between pulses; if we choose the lowest operating frequency to be 1 kHz, then:
C6 » 10-3/(3.9 x 103),
so: C6 » 0.256 µF
Recalling that we require: C6 > 20.C5:
Let: C6 = 4.7 µF
k = 220 x 10-12/(220 x 10-12+ 4.7 x 106) = 46 x 10-6
Since k is very small, the output ripple is very small (which is a good thing) and although the above calculations used Vomin, this value will be extremely close to the average which is what a meter or oscilloscope would measure. Since the calibration is determined by R13 the meter or 'scope should have an input impedance very much greater than R13. Alternatively, the input impedance could be taken into account when calculating R13. The best solution, if a low impedance load were used, would be to buffer the output with an op-amp in a voltage-follower configuration.
Performance
Minimum input for reliable operation: 0.7V (pk-pk) or 0.25V rms, measured at 50 kHz.
The ouput voltage is linearly related to input frequency from 200 Hz to over 800 kHz with a slope of 5.15 Volts/MHz (passing through the origin).