feedback resistance การใช้

• For a good noise performance, a high feedback resistance should thus be used.
• The feedback resistance and therefore the sensitivity are thus limited by the required operating frequency of the transimpedance amplifier.
• However, a larger feedback resistance increases the output voltage swing, and consequently a higher gain from the operational amplifier is needed, demanding an operational amplifier with a high gain-bandwidth product.
• If a very high output impedance is required, operating in common base provides a solution, because it stops the negative feedback; e ) because common base interposes a ground between this notional feedback resistance and the input, the performance of the output circuit is less affected by changes in input circuit impedance-this is sometimes useful.
• However teh configuration is usefull because a ) the output is at higher impedance than the input and so there is voltage gain ( and therfore power gain ); b ) it is usefull in sitautions where the signal source is low impedance, and the following circuitry is relatively high impedance; and c ) the configuration puts a signal ground between the the collector-base capacitance and input, improving high frequency performance; d ) there is a notional feedback resistance ( Early effect ) between collector and base that causes negative feedbakc and thereby lowers the output resistance in the common emitter configuration.