Transistor base current

[Kevin Weddle]

How much change in transistor current should there be for best signal quality? I'm using a 741 opamp input 100mVpp. I think it's DC biased at 100nA. Should I use a smaller input signal or is 100mVpp a good size?

 

[Hero999]

A schematic would be nice.

 

[audioguru2]

A 741 opamp is 42 years old and was not made for quality, it was made to be the first opamp with an internal frequency compensation capacitor.
Its spec's are at a supply of plus and minus 15V, not with lower voltages.
Its output has poor response above only 9kHz at full output (28V p-p).
It is fairly noisy (hiss).

Its input bias current is typically 80nA but could be as high as 500nA.
Its input transistors are emitter-followers and have a common-mode voltage range of at least plus and minus 12V when the supply is plus and minus 15V.

 

[Kevin Weddle]

A common mode voltage range? You mean low noise.

 

[audioguru2]

The input common-mode voltage range of the inputs of an opamp are the range of voltages that the input transistors work properly. Most opamps have inputs that work when their voltage is between 3V above the negative supply voltage to 3V below the positive supply voltage.

The LM358 dual and LM324 quad low power "single supply" opamps use PNP darlington input transistors so their input common-mode voltage range includes the negative supply voltage which can be 0V.

The TL07x and TL08x opamps use P-channel Jfets as the input transistors so their input common-mode voltage range includes the positive supply voltage.

Low noise is a completely different specification.

 

[Kevin Weddle]

That just means the opamp is functional and can withstand the voltage level. Opamps are normally biased midpoint of the supply voltage. If you operate the opamp at some other common mode voltage, you may get a better output. Discrete transistors operate ideal at their own bias currents. I think a higher transistor current allows for safer biasing, but you have to be within circuit constraints.