Hi guys, I am learning, so, if you will spank me, do it softly...
I am trying to design a common-emitter amplifier just for the whole purpose of learning.
Can you guys see if the calculations I have done are correct and help me calculate the input/output coupling capacitors?
The circuit will be like this:
I have chosen the following values: Vcc = 18, Ic = 8mA, transistor 2N2222A.
hfe for that current is 225, according to this: http://web.mit.edu/6.101/www/reference/2N2222A.pdf
I have chosen Ve to be 1V.
Vce minimum is chosen to be 2V. I see it is not good to have a Vce lower than that.
If Vce minimum = 2V, them Vc minimum = Vcemin + Ve = 2 + 1 = 3V
So, the output may swing from 3 to 18 V, right?
The middle of this range is 10.5V. So I have designed Vc to be biased ad 10.5V.
For maximum output,
=================
CALCULATING RE
=================
ie = ic + ib
ie = 8mA + 8mA/225
ie = 8.0355 mA
if Ve = 1
and Ve = Re.ie
then
Re = 1/8.0355mA = 124.44 Ω
=================
CALCULATING RC
=================
If Vc is set at 10.5V and Vcc is 18V,
Rc should produce a voltage drop of
18 - 10.5 = 7.5V
so,
Vrc = 7.5 = Rc.ic
7.5 = Rc(8mA)
Rc = 937.5 Ω
=================
CALCULATING R1 and R2
=================
To make the circuit stable I choose i1 to be 10% of iC
so,
i1 = 10% of 8ma = 800 uA.
if Ve = 1 and Vbe = 0.7V, then Vb = 1.7V
__________________
i2 = i1 - ib
R2.i2 = Vb
R2 (i1 - ib) = Vb
R2 = Vb/(i1 - ib)
R2 = 1.7/(800uA - 35.55uA)
R2 = 2223 Ω
_________________
R1 = (Vcc - Vb)/i1
R1 = (18 - 1.7)/800uA
R1 = 20,375 Ω
________________
=================
CALCULATING Cin
=================
First I calculate the input impedance that, as far as I know is equal to
Zin = R1 // R2 // (rπ + Re(hfe + 1))
rπ = kT/qib = 26mV/35.55uA
rπ = 727.09 Ω
So,
Zin = 20,375 // 2223 // 727.09
Zin = 1874 Ω
_____________
As far As I have researched, Cin is calculated to have a reactance equal to 2/3 of Zin at the cut frequency. Suppose 20Hz.
Xc = 2/3 of Zin = 1249 Ω
So
Cin = 1/2πfXc
Cin = 1/2π(20)(1249)
Cin = 6.36uF
___________________
Now I don't know how to calculate Cout. Suppose the collector will drive a 600Ω load (high impedance headphone).
___________________
Two questions:
1) are my calculations correct
2) how do I calculate Zout and consequently Cout, to be connected at the collector?
Thanks.
I am trying to design a common-emitter amplifier just for the whole purpose of learning.
Can you guys see if the calculations I have done are correct and help me calculate the input/output coupling capacitors?
The circuit will be like this:
I have chosen the following values: Vcc = 18, Ic = 8mA, transistor 2N2222A.
hfe for that current is 225, according to this: http://web.mit.edu/6.101/www/reference/2N2222A.pdf
I have chosen Ve to be 1V.
Vce minimum is chosen to be 2V. I see it is not good to have a Vce lower than that.
If Vce minimum = 2V, them Vc minimum = Vcemin + Ve = 2 + 1 = 3V
So, the output may swing from 3 to 18 V, right?
The middle of this range is 10.5V. So I have designed Vc to be biased ad 10.5V.
For maximum output,
=================
CALCULATING RE
=================
ie = ic + ib
ie = 8mA + 8mA/225
ie = 8.0355 mA
if Ve = 1
and Ve = Re.ie
then
Re = 1/8.0355mA = 124.44 Ω
=================
CALCULATING RC
=================
If Vc is set at 10.5V and Vcc is 18V,
Rc should produce a voltage drop of
18 - 10.5 = 7.5V
so,
Vrc = 7.5 = Rc.ic
7.5 = Rc(8mA)
Rc = 937.5 Ω
=================
CALCULATING R1 and R2
=================
To make the circuit stable I choose i1 to be 10% of iC
so,
i1 = 10% of 8ma = 800 uA.
if Ve = 1 and Vbe = 0.7V, then Vb = 1.7V
__________________
i2 = i1 - ib
R2.i2 = Vb
R2 (i1 - ib) = Vb
R2 = Vb/(i1 - ib)
R2 = 1.7/(800uA - 35.55uA)
R2 = 2223 Ω
_________________
R1 = (Vcc - Vb)/i1
R1 = (18 - 1.7)/800uA
R1 = 20,375 Ω
________________
=================
CALCULATING Cin
=================
First I calculate the input impedance that, as far as I know is equal to
Zin = R1 // R2 // (rπ + Re(hfe + 1))
rπ = kT/qib = 26mV/35.55uA
rπ = 727.09 Ω
So,
Zin = 20,375 // 2223 // 727.09
Zin = 1874 Ω
_____________
As far As I have researched, Cin is calculated to have a reactance equal to 2/3 of Zin at the cut frequency. Suppose 20Hz.
Xc = 2/3 of Zin = 1249 Ω
So
Cin = 1/2πfXc
Cin = 1/2π(20)(1249)
Cin = 6.36uF
___________________
Now I don't know how to calculate Cout. Suppose the collector will drive a 600Ω load (high impedance headphone).
___________________
Two questions:
1) are my calculations correct
2) how do I calculate Zout and consequently Cout, to be connected at the collector?
Thanks.