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This simple digital probe uses a differential amplifier consisting of two NPN transistors T2 and T3 and two field-effect transistors T1 and T4 to obtain a high-impedance input.
English (United States)
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Digital Probe Head
This simple digital probe uses a differential amplifier consisting of two NPN
transistors T2 and T3 and two field-effect transistors T1 and T4 to obtain a high-
This differential amplifier compares a logic signal at input IN with a threshold
voltage V(REF). If the input voltage differs from the threshold voltage by more
than 300 mV, the differential amplifier goes into saturation, operating in the digital
mode. At a positive input voltage, T2 is conductive and T3 is switched off,
whereas a negative input voltage causes T3 to become conductive and T2 to be
The probe is protected against overvoltages of up to + 100 V in the following
A positive overvoltage, although leading to a breakthrough of the emitter-
base diode of transistor T3, is limited by the voltage across input resistor R2.
The breakthrough current is limited by R9, so that transistor T3 is not damaged.
A negative overvoltage causes a breakthrough of the gate-source diode of
T1. As the breakthrough current is limited by R2 and R5, T1 is not damaged.
To obtain a low input capacitance, the protection circuit does not use a
clamping diode. To prevent the gate-drain capacitance of T1 and the input
capacitance from adding up, the two capacitances are decoupled by R4. As a
result, the input capacitance is less than 3 pF, and a DC bandwidth of up to 300
Mc/s is obtained. The input resistance measuring, for example, 1 Mohm, is
determined mainly by R1. C...