1. The problem statement, all variables and given/known data
I'm studying a circuit with BJT's and I'm asked to determine the input resistances of the two amplification steps of the circuit. The circuit I'm analyzing is the following one:
2. Relevant equations
3. The attempt at a solution
To determine the input resistances I performed an AC analysis and obtained the following results:
I know that the values are not perceptible but they are Rin1=23.272231 kohm and Rin2=220.77106 kohm.
I decided to choose a somewhat random value in the zone of the graphic where the curves are constant (I'm not sure if this is correct...).
Well this value are very different from what I obtained when I analyze the theoretical values of the input resistances.
Using:
$$R_{i2}=(β+1) R_{E3}//R_L+r_{π2}$$
where r_{π2} is 9.375 kohm and beta is 300.
I obtained R_i2=240.375 kΩ
And for Ri1
$$R_{i1}=R_1//R_2//(r_π1+R_{E1} (1+β))$$
I obtained R_i1=20.183kohm.
I used the smallsignal analysis models to determine this...
Now this is odd. I'm getting an error of about 15% on Ri1 and 7% on Ri2. Am I doing something wrong or are this deviations perfectly normal and dependent on the methods used on the theoretical analysis and on LTSpice. Can someone help me clarify this?
I'm studying a circuit with BJT's and I'm asked to determine the input resistances of the two amplification steps of the circuit. The circuit I'm analyzing is the following one:
2. Relevant equations
3. The attempt at a solution
To determine the input resistances I performed an AC analysis and obtained the following results:
I know that the values are not perceptible but they are Rin1=23.272231 kohm and Rin2=220.77106 kohm.
I decided to choose a somewhat random value in the zone of the graphic where the curves are constant (I'm not sure if this is correct...).
Well this value are very different from what I obtained when I analyze the theoretical values of the input resistances.
Using:
$$R_{i2}=(β+1) R_{E3}//R_L+r_{π2}$$
where r_{π2} is 9.375 kohm and beta is 300.
I obtained R_i2=240.375 kΩ
And for Ri1
$$R_{i1}=R_1//R_2//(r_π1+R_{E1} (1+β))$$
I obtained R_i1=20.183kohm.
I used the smallsignal analysis models to determine this...
Now this is odd. I'm getting an error of about 15% on Ri1 and 7% on Ri2. Am I doing something wrong or are this deviations perfectly normal and dependent on the methods used on the theoretical analysis and on LTSpice. Can someone help me clarify this?
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