Single Stage Common Emitter Amp Design

[charkins]

Homework Statement

I am designing a voltage divider common emitter amplifier given the following parameters

Av= 40 dB +/- 1dB
Zi> 1000 ohms
Zout< 5000 ohms
Stability factor = 5
Beta*re= 650 ohms
Beta minimum = 35
Beta maximum = 300
Vcc = 20 volts
Rload = 33 kilo-ohms
Rsource = 470 ohms
Vce= 10 volts
Ve= 2 volts
We can use the approximation that Ic=Ie
My teacher has given us a "hint" saying to work backwards from the AC analysis.
I need to solve for Re, R1, R2, Rc and the respective currents.

I am lost as to where to start with this circuit.

2. Homework Equations [/b
Ic=Ie
Ib=Beta*Ic
Vb=(Vcc(R2))/(R1+R2)
Vcc-Vce-Ic(Re+Rc)=0
re=(26*10^-3)/Ic
Stability Factor=Rthevenin/Re
Av in db=20log(Vo/Vi)

The Attempt at a Solution

I am completely lost where to start with this circuit. So I have not attempted it.

 

[aralbrec]

This sounds like a discrete design using a voltage divider to bias Vb, Rc and Re to help bias and set gain, capacitor on the emitter to bypass Re for common emitter small signal gain, capacitor coupled source attached to the base and capacitor coupled load at the collector.

But it's speculation until we can see a circuit diagram.

 

[charkins]

http://tinypic.com/view.php?pic=2ew1t0l&s=6

I believe this is how the circuit should be constructed

I just cannot figure out how to solve for the missing values with the given parameters.

 

[aralbrec]

You have quite a bit of information given, some of it is ac performance and some of it is dc bias conditions. You will need to calculate those specifications in terms of circuit components, which will lead to simultaneous constraints on the component values.

The only way to find these constraints is to start writing equations. As a first step, I would look at the dc circuit and try to determine the ac transistor model parameters. You have a few explicit dc bias conditions: Vce = 10 volts, Ve = 2 volts. This also specifies the dc voltage across Rc. If you know the dc current, you have Rc and Re specified. One thing that grabs my attention is that βr_e = 650 ohms. This is because r_e depends on the bias current so you may be able to use this information to find the dc bias current. Now β is always statistically variable so this is not the best way to find the dc bias current if there is another way. The other use for βr_e is in computing ac input impedance since this will be the value of r_e reflected into the base side of the circuit (β>>1).

With the dc voltage at the emitter, you know the voltage at the base, which let's you find a ratio for R1 and R2. From there you will have to look at the ac model. The ac input impedance will place a second constraint on R1 and R2, which (given Re and βr_e), you will be able to solve. If you couldn't find Re before, the output impedance may place a second needed constraint on Rc, which will specify Re from dc bias conditions. Actually that may be one of the first steps to find Rc and Re.

As you can see, you need to write down the equations and keep in mind what the specifications are telling you about component constraints.

I see you've moved onto a JFET in similar configuration so maybe you've left this one already but the problem is almost the same.

 

[rezeew]

But I can provide some insights on how to approach this design problem.

First, we need to understand the basic concept of a single stage common emitter amplifier. It consists of a transistor, resistors, and capacitors, and it is used to amplify a small input voltage signal.

To design this amplifier, we need to consider the given parameters such as Av, Zi, Zout, stability factor, Beta*re, Beta minimum and maximum, Vcc, Rload, Rsource, Vce, and Ve. These parameters determine the performance and characteristics of the amplifier.

One way to approach this design problem is to work backwards from the AC analysis, as suggested by your teacher. This means starting with the desired Av and using the equation Av in db=20log(Vo/Vi) to calculate the ratio of output voltage to input voltage. From this, we can determine the gain of the amplifier.

Next, we can use the stability factor equation to calculate the value of Re. Then, using the equation Vcc-Vce-Ic(Re+Rc)=0, we can solve for the collector current (Ic). Knowing the value of Ic, we can calculate the base current (Ib) using the equation Ib=Beta*Ic.

Moving on to the input side of the amplifier, we can use the voltage divider equation Vb=(Vcc(R2))/(R1+R2) to calculate the base voltage (Vb). From this, we can determine the values of R1 and R2.

Finally, using the equation re=(26*10^-3)/Ic, we can calculate the value of re, which is the dynamic resistance of the emitter diode. This will help us determine the values of Rc and the respective currents.

In summary, to design a single stage common emitter amplifier, we need to start with the desired Av and work backwards to determine the values of Re, R1, R2, Rc, and the respective currents. It may also be helpful to simulate the circuit using a software program to verify the design.