# Electronic – Common emitter bjt amp Q point with and without current mirror

amplifierbjtcurrent-source

What is the real benefit of voltage divider CE BJT amplifier schematic like that? If I use different BJT's in real life with various betta I'll give different voltage drops XMM1. Different betta of real BJT's willn't allow to center the Q-point voltage XMM1 on output curve. Or it's not critical to allow Q-point some drift on output curve?

I'm also tryring to use current source instead R3 to fix Q1 collector current but I can't understand is that schematic usable in real life? And I can't understand how to select R3, R4 values if I know Q1 collector current (that equal to I_ref tuned by R1)?

• In the simple CE configuration you provide as the first diagram, it is common to design it without over-dependence on the \$\beta\$ of the BJT. As a result of that approach, it usually turns out that variations of \$\beta\$ between parts in the same family (at worst, on the order of \$\le 50\%\$) has only a relatively small impact on the quiescent point, so long as you've made your voltage divider "stiff" enough. What impact there is, is due to the variations of base current as a loading on the voltage divider's Thevenin resistance. So this is quite predictable, too.
• In the simple CE configuration you provide as the first diagram, it is also common to design it without over-dependence on the \$V_{BE}\$ of the BJT. (\$V_{BE}\$ variations are due to saturation current variations, which themselves can vary by a factor of as much as 3 between parts. But since this is part of a logarithm function, the impact is less than you may imagine at first.) This is achieved usually for a different reason -- temperature stability. But it also has another impact, which is to reduce dependence of the quiescent point on \$V_{BE}\$ variations. This is often the larger impact on the quiescent point, though, than variations of \$\beta\$ -- though of course it all depends upon the design goals and the actual design choices that are made.