Field-effect transistor in a pulse stretcher circuit
Abstract Generated abstract
This paper describes an electronic pulse stretcher circuit designed to improve recording of betatron pulse-parameter drift with devices such as loop oscillographs. The authors propose a hybrid semiconductor arrangement using a field-effect transistor in a common-drain output stage, allowing a small memory capacitance, reduced error for short input pulses, and long storage with limited droop. The circuit stretches negative input pulses as short as 0.1 microsecond to about 5000 microseconds, with less than 1 percent undercharge or peak-drop error under the stated conditions. Reported amplitude characteristics show a transfer coefficient in the 0.5 to 10 V range with integral nonlinearity below 3 percent and little dependence on input-pulse duration.
Full Text
FIELD-EFFECT TRANSISTOR IN A PULSE STRETCHER CIRCUIT
V. A. Moskalev, Yu. A. Gromov
When recording the drift of the pulse parameters of betatrons, which is necessary for statistical studies of their operation, there arises a need to increase the duration of the pulses arriving at the recording device, for example, a loop oscillograph [1]. For this purpose, circuits of electronic pulse stretchers are used, to which the following requirements are imposed:
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The possibility of stretching pulses with a duration of \(1\ \mu\text{s}\) and less.
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Stretching pulses to a duration comparable with the operating period of the accelerator, which will increase the efficiency of recording.
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Minimal droop of the top of the stretched pulse, which affects the sharpness of the image and facilitates the reading of information from the carrier (photographic film). The requirements of items 2 and 3 can be combined into one: the requirement of a large stretching (expansion) factor on the order of \(10^3 \div 10^4\).
From the theory of pulse stretchers or expanders, widely covered in [2], it follows that a large expansion factor can be obtained by means of tube circuits; in expanders made entirely with semiconductor devices, without special measures that complicate the circuit, this cannot be achieved. However, compared with tube expanders, semiconductor expanders possess a number of valuable qualities, the main one being the possibility of obtaining small errors when expanding pulses of short duration \((<1\ \mu\text{s})\).
The proposed expansion circuit, intended for operation with a loop oscillograph, in our opinion has the positive qualities of both semiconductor and tube expansion circuits. This is achieved through the use of a field-effect transistor at the output of the circuit. The operation of the circuit (Fig. 1) is as follows.
An input pulse of negative polarity, through the emitter follower \((T_1)\) and the fast-acting diode \(D_1\), charges the memory capacitance \(C_2\) to the amplitude value. In this case, a small charging time constant is obtained, and the amplitude of the pulses at the output is practically independent, over a wide range, of the duration of the pulses at the input.
Diode \(D_2\) is normally open. Therefore the memory capacitance can be charged only if diode \(D_2\) is closed. It is closed for 5000 μsec by a control pulse from the one-shot multivibrator (\(T_4, T_5\)), triggered by the leading edge of the input pulse. Thus, on the capacitance a pulse is formed with a duration of 5000 μsec and with an amplitude corresponding to the value of the amplitude of the input pulse. This value of the duration of the pulse being stretched was chosen from the condition of increasing the recording efficiency and exceeding the time constant of comparatively low-power oscilloscope cables.
Fig. 1. Pulse stretcher circuit
To substantially reduce the memory capacitance \(C_2\), which leads to a reduction in the minimum duration of the pulses being stretched \(t_{u\min}\), and also to increase the storage time \(T_{\text{st}}\), a field-effect transistor \(T_2\) is included in the measuring circuit in a “common-drain” configuration. The coupling to the capacitance is galvanic. The field-effect transistor makes it possible, practically without a drop in the peak value, to transfer the pulse formed on the capacitor to the power-amplifier circuit (\(T_6, T_7, T_8\)) for matching to the low-resistance cable impedance.
The circuit has the following characteristics:
\(t_{u\min} = 0.1\) μsec and \(T_{\text{st}} > 5000\) μsec at an input-pulse amplitude \(U_{\text{in}} = 5\) V.
(The minimum pulse duration and the storage time are taken to be the duration of the input pulse at an allowable error due to undercharging of the memory capacitance equal to 1%, and the time at which the drop in the peak of the stretched pulse will be equal to 1%.)
The amplitude characteristic of the stretcher for input-pulse durations \(t_{u\text{in}} = 0.4;\ 10;\ 100\) μsec is presented in Fig. 2. It follows from it that the transfer coefficient of the circuit in the range 0.5–10 V has an integral nonlinearity of less than 3% and does not depend on \(t_{u\text{in}}\).
Thus, with the aid of the circuit described above, it is possible to obtain a simple, compact device with good characteristics,
which makes it possible to make wide use of multichannel recording instruments, such as a loop oscillograph, for statistical studies of betatron operation, as well as of a number of other accelerators.
Visible labels in the figure: \(U_m\) output; \(U_m\) input; \(t_i = 0.5,\ R = 100\ \mathrm{M\Omega}\).
Fig. 2. Amplitude characteristic of the pulse stretcher
REFERENCES
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V. A. Moskalev, Yu. A. Gromov. Apparatus for recording pulsed parameters of a betatron. Izv. TPI, vol. 193, Tomsk, 1970 (in press).
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V. V. Matrachev. Voltmeters for Single Pulses. Moscow, Energiya Publishing House, 1967.