[time-nuts] how to find low noise transistors

[Charles Steinmetz]

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