which amplifier design is good and what is different?

[karthickb3e]

hi one and all!
i need gain value of 1000. so i go for 2 stage amplifier ...... i got confusion in design references,
design 1

design 2:

in design 1:
2 stage is also given additional bias using R6 and R7....

but in design 2:
2 stage is biased from 1 st stage output .....

my question is additional voltage divider bias on 2nd stage is important or not....


if not putting it, may be happen any other problems or not........ which design is better?

please help ...!

 

[Pjdd]

The matter of which one is better depends partly on how the component values are chosen. In general, the second one has the advantage of using fewer components and better low frequency response. Separate biasing is not essential. The absence of a voltage divider in the second stage will not cause any problem as long as proper resistor values are chosen.

In both designs, the gain of each stage is set separately. In practice, it's often better to reduce the amount of negative feedback for each stage and thus have a high overall gain, and then give overall negative feedback to set the required gain. This can be done by partly or fully bypassing the second emitter resistor with a capacitor and then inserting a feedback resistor from T2 collector to T1 emitter.

 

[karthickb3e]

thank you....how to select values of Rc2 and Re2... is there any formula...


also i have one doubt that u suggest to put feedback resistor from T2 collector to T1 emitter....
i seen one circuit which uses the feedback resistor from T2 emitter to T1 base ....

 

[FvM]

With design 1, you have good chances to achieve the intended gain, with design 2, you'll most likely fail to build an amplifier with stable bias. But the circuit isn't complete, at least input and output coupling capacitors are missing.

 

[milind.a.kulkarni]

I think in design-2 you should have voltage devider network need to be added at second stage....as so better to put decouling capacitors and eimmter bypass capacitor....not the values of RC and RE are based on dc load line concept....

refer following -

https://www.physics.ohio-state.edu/~durkin/phys617/commonemitter.pdf

CE Amplifier design - All About Circuits Forum

Good Luck

 

[Pjdd]

Since T1 collector is directly connected to T2 base, their voltages are the same and T2 emitter is about 0.6V lower than that. Ie2 = Ve2/Re2 ≈ Ic2. Choose Rc2 so that the voltage drop Ic2*Rc2 from Vcc will leave enough Vce for proper transistor operation.

Let's take an example. Say Vcc = 12V and the biasing of stage 1 is such that Vc1 = 2.6V. Then Ve2 will be about 2V. If Re2 = 1k, then Ie2 = 2mA ≈ Ic2. If Rc2 = 2.7k, the voltage drop = 5.4V and Vc2 = 6.6V and Vce2 = 4.6V.

We have already chosen Vc1 = 2.6V. If Rc1 = 10k, then Ic1 = (12-2.6)V/10k = 0.94mA ≈ Ie1.
If Re1 = 470Ω, then Ve1 = 0.4418V. Using 0.6V for Vbe1, Vb1 = 1.0418V.
If R2 = 10k, the current through it = 104.18µA
Add about 4µA for Ib1 (for beta ~250), then the current through R1 = 108.18µA
The voltage drop across R1 = Vcc - Vb1 = (12 - 1.0418)V = 10.9582V
R1 = 10.9582V/108.18µA = 101.3k, say 100k.

---------- Post added at 14:54 ---------- Previous post was at 14:39 ----------

The design example above should be stable enough. It's just one possible combination of bias and component values and other combinations can be used. As mentioned before, bypassing Re2 is important to have enough open-loop gain. Splitting Re1 into, say 390 + 82 ohms instead of a single 470 ohms and bypassing the 390 ohms will also result in more open-loop gain and leave a wider margin for the negative feedback factor.

You can have an improved bias stability if the upper terminal of R1 is fed from T2 emitter instead of Vcc. For that, bypassing Re2 is a must. The value of R1 will also have to be changed since it will now be fed from a lower voltage.