Rick wrote:
OTOH, the other cure for high base spreading resistance is to simply parallel multiple devices. This avoids the bad side effects you mention.
Yes, if you can operate each of the transistors at the same current you would have used for a solo transistor.
Typically, though, you need to reduce the current through each transistor so that the total current remains approximately the same as you would use for one transistor, in which case you do not escape the side effects. There is a practical limit to increased total current in circuits that use collector resistors to transforn the devices' transconductance into voltage gain -- as you increase the device current, you need to reduce the collector resistors (and, therefore, stage gain) or increase the power supply voltage (which gets out of hand in a hurry). In some cases, this can be avoided by using current source or current mirror collector loads. Also, when paralleling transistors, you need to allow for the noise of any emitter ballasting resistors that may be necessary to equalize currents between the transistors.
All is not lost, however -- the parallel device strategy encourages the designer to use multiple, lower current devices to replace a single, higher current device (for example, using multiple 2N5179s to replace a single 2N5109).
Some '80s hi-fi preamps went to absurd lengths with parallel input devices. A few had 20 or so devices in parallel for each of the four input devices of a complementary differential pair (x2 for stereo, so 160 input transistors for just one stereo stage). But in order for the input bias and offset currents to remain somewhat reasonable, and for other practical reasons, the current per device had to be reduced to the point that the bad side effects were glaringly apparent. (One more all-too-common example of a designer being blindly committed to his or her Good Idea and pursuing it to the detriment of the product.)
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
