282: Difference between revisions
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an input impedance of 1 megohm in parallel with 16.5 picofarads and a low output impedance. | an input impedance of 1 megohm in parallel with 16.5 picofarads and a low output impedance. | ||
The high-impedance input stage is a dual-JFET where transistor A operates as a source-follower | 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 | 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, | 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 | the signal is fed to a Sziklai pair formed of a PNP transistor first, driving an NPN output | ||
Revision as of 11:40, 17 March 2012
The Tektronix Type 282 is an active adapter introduced in 1966 that allows high-impedance probes to be used with instruments that have 50-ohm inputs. It is a unity-gain buffer with an input impedance of 1 megohm in parallel with 16.5 picofarads 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 risetime is 3ns when driven from a 25-ohm source.
