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The '''Tektronix Type 282''' is an active adapter [[introduced in 1966]] that allows high-impedance probes to be used with instruments that have 50 Ω inputs. | The '''Tektronix Type 282''' is an active adapter [[introduced in 1966]] that allows high-impedance probes to be used with instruments that have 50 Ω inputs. | ||
It is a unity-gain buffer | It is a unity-gain buffer with an input impedance of 1 MΩ in parallel with 16.5 pF and a low output impedance. | ||
The high-impedance input stage is a dual-JFET where transistor A operates as a source-follower and transistor B operates as a current source biased with V<sub>gs</sub> = 0, maintaining a constant (I<sub>dss</sub>) current through transistor A, independent of input signal voltage. After the FET input stage, the signal is fed to a Sziklai pair formed of a PNP transistor first, driving an NPN output transistor. The Sziklai pair is non-inverting and has a voltage gain greater than unity, which allows the overall gain of the 282 to be trimmed for exactly unity gain. The rise time is 3 ns when driven from a 25 Ω source. | The high-impedance input stage is a dual-JFET where transistor A operates as a source-follower and transistor B operates as a current source biased with V<sub>gs</sub> = 0, maintaining a constant (I<sub>dss</sub>) current through transistor A, independent of input signal voltage. After the FET input stage, the signal is fed to a Sziklai pair formed of a PNP transistor first, driving an NPN output transistor. The Sziklai pair is non-inverting and has a voltage gain greater than unity, which allows the overall gain of the 282 to be trimmed for exactly unity gain. The rise time is 3 ns when driven from a 25 Ω source. | ||
Latest revision as of 11:41, 28 November 2023
The Tektronix Type 282 is an active adapter introduced in 1966 that allows high-impedance probes to be used with instruments that have 50 Ω inputs.
It is a unity-gain buffer with an input impedance of 1 MΩ in parallel with 16.5 pF and a low output impedance.
The high-impedance input stage is a dual-JFET where transistor A operates as a source-follower and transistor B operates as a current source biased with Vgs = 0, maintaining a constant (Idss) current through transistor A, independent of input signal voltage. After the FET input stage, the signal is fed to a Sziklai pair formed of a PNP transistor first, driving an NPN output transistor. The Sziklai pair is non-inverting and has a voltage gain greater than unity, which allows the overall gain of the 282 to be trimmed for exactly unity gain. The rise time is 3 ns when driven from a 25 Ω source.
Power Supply
The 282 internally is supplied with ±10 V stabilized with Zener diodes. This allows the 282 to be compatible with the different negative supply voltages being present on the B pin of the probe power connector ranging from -12.2 V (3S1) to -25 V (4S1). Positive supply is from +100 V.
Compatible probe power sampling plug ins
The 282 MOD 125D (015-0146-00) is a modified version which is supplied by a 015-0073-00 ±12 V power supply. Using a 015-0147-00 tee connector, two 282 MOD 125D units may be supplied.
Pictures
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282 MOD 125D with 015-0073-00 and 015-0147-00 in 1970 catalog
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