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Hello guys, this is my very first question.

This is a part from a BJt audio amplifier volume control with 3 stages (Pre-amplifier,Buffer, & Power amplifier). This is the power amplifier stage.

I am asked to design the resistors RB3b & RB3a to provide a bias current of Ic=(375/20) mA when the input and output terminals are open circuits and to get Rin.

I proceeded in the problem,The voltage drop between the two diodes is 1.4V so I found that the voltage drop between each resistor is 3.8V and Vce for each transistor is 4.5V.

The problem is I cannot find beta (current gain) from the data sheet to deduce Ib and the current in the resistors. the date sheet provide beta for maximum of Vce=1V so I am stuck.

plus I read that the current in the resistors should be 10 Ib, is that right? I would appreciate any help, thank you.

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  • \$\begingroup\$ Job interview? Or classwork? This cannot possibly be a realistic design question. \$\endgroup\$
    – jonk
    May 30, 2020 at 0:05
  • \$\begingroup\$ @jonk class project, can you help ? \$\endgroup\$
    – Amr Ebrahim
    May 30, 2020 at 0:07
  • \$\begingroup\$ Are you allowed to modify the schematic? Or are you absolutely stuck with that crap? \$\endgroup\$
    – jonk
    May 30, 2020 at 0:09
  • \$\begingroup\$ @jonk I am certainly stuck \$\endgroup\$
    – Amr Ebrahim
    May 30, 2020 at 0:12
  • \$\begingroup\$ What's the worst-case peak-to-peak range of the signal driving this stage? (Hopefully, as close to zero as possible. This output stage topology works best when there's no signal.) \$\endgroup\$
    – jonk
    May 30, 2020 at 0:13

1 Answer 1

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The problem is I cannot find beta (current gain) from the data sheet to deduce Ib and the current in the resistors. the date sheet provide beta for maximum of Vce=1V so I am stuck.

At low Collector current the current gain doesn't vary much above saturation, so you can take the value at 1 V and it should be close enough. The Base-Emitter voltage drop may be more important. Here's a graph of IC vs VBE from the Vishay BC337/338 datasheet.

enter image description here

At 25 °C and IC = 20 mA, VBE is ~0.65 V. Note the strong temperature dependence. This is why you need to use a diode to set the bias voltage, and it should be thermally coupled to the transistor so their voltages track.

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