Improving grounded emitter amplifier with collector-to-base bias

Question

According to books, this simple grounded-emitter amplifier has some shortcomings. For example, 1) the d.c. operating point is unstable. 2) since the intrinsic resistance of emitter, \$r_\text{e}\$, changes with collector current, the voltage gain of the circuit is not constant, leading to distortion. The books say that a very small change improves the circuit: one side of R1 should be connected to the collector instead of Vcc. I understand why this improves the stability (and the books mainly discuss this), but how does this affect the other shortcomings of the initial grounded-emitter amplifier?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

The BJT collector-base negative feedback (NFB) current is the preferred way to linearize a BJT amplifier and reduce distortion near clipping and saturation or improve THD by reducing the variation in Vbe.

• There is a tradeoff such that voltage gain is reduced for this improvement, and input and output impedance is also reduced by the ratio of NFB.

• It reduces the effects of wide variations in hFE

• The fixed bias R may be pull-up or pull-down in order to centre the collector voltage DC operating point depending on Vcc & Rc.

  • e.g. If you eliminate input Cap, use pull-up R.

• NFB lowers the input and output resistance which depends on gain available

• You can change the input impedance and hFE on this simulation and see the effects.

• Just as an emitter R reduces the Vbe variation and reduces gain, so too does collector feedback with voltage open loop gain and closed loop.

• There are rigorous equations on this site for this BJT NFB voltage gain. (somewhere)

Answer 2

Both circuits (as drawn and implied) have a common problem of not being able to capably control voltage gain: -

The 2nd circuit is definitely better but, like an inverting op-amp without an input resistor, the voltage gain is maxed out and the input impedance is quite small. However, the first circuit is pretty much poor in terms of DC stability compared to when using an emitter degeneration resistor as per this Q and A. You would never use the 1st circuit without some overall feedback control (compared to emitter degeneration).

So, it's probably better to compare the collector-to-base feedback circuit (plus an added input resistor) with, a standard circuit using an emitter resistor: -

I've adjusted the input resistor R8 to give the same dc collector voltage of 6 volts (half of Vcc) and I've adjusted the emitter degeneration resistor to produce the same AC signal gain. So, with a small input voltage of 0.1 volts peak at 1 kHz, we see near identical performance: -

I've plotted Va (left circuit using emitter resistor), Vb (right circuit with collector-base feedback) and the input voltage multiplied by circuit gain, inverted and added to 6 volts.

It's a dead heat as far as I'm concerned. However, with a higher amplitude signal we begin to see some subtle differences: -

With an even higher input signal: -

It's swings and roundabouts really and the strong conclusion I have is this: -

• you need an input resistor to stabilize gain (so forget about your first circuit)
• pretty much you can use emitter degeneration or collector-base feedback
• if you prefer to have a more symmetrical clipping effect then use the collector-base feedback resistor
• if you want your signal output to get close to 12 volts then use emitter degeneration