Question

7. [20%] The circuit below is a BJT differential amplifier with $V_{CC} = V_{EE} = 3 V$. The DC current source $I_{EE}$ is assumed ideal. The two transistors are identical, operate in the active region, and have $alpha = 0.99$ and $V_A = infty$. The thermal voltage $V_T$ is 26 mV at room temperature. The input is purely differential, i.e., $V_{ip} = -V_{im} = V_{id}/2$. a) Draw small-signal differential-mode half circuits of the differential amplifier. (5%) b) Express the differential voltage gain $A_d$, which is defined as $(V_{op} - V_{om})/V_{id}$, in terms of $I_{EE}$, $R_L$, $alpha$ and $V_T$. (6%) c) If $I_{EE} = 2mA$, what should be the value of $R_L$ to achieve $A_d = 40$? (4%) d) Choose another set of $I_{EE}$ and $R_L$ to maintain $A_d = 40$ while reducing the power consumption by half compared to the situation in part c). (2%) e) What is the maximum $A_d$ that the circuit can achieve if the DC levels at the output nodes $V_{op}$ and $V_{om}$ must be equal to or higher than 0V? (3%)

          7. [20%] The circuit below is a BJT differential amplifier with $V_{CC} = V_{EE} = 3 V$. The DC current source $I_{EE}$ is assumed ideal. The two transistors are identical, operate in the active region, and have $alpha = 0.99$ and $V_A = infty$. The thermal voltage $V_T$ is 26 mV at room temperature. The input is purely differential, i.e., $V_{ip} = -V_{im} = V_{id}/2$.
a) Draw small-signal differential-mode half circuits of the differential amplifier. (5%)
b) Express the differential voltage gain $A_d$, which is defined as $(V_{op} - V_{om})/V_{id}$, in terms of $I_{EE}$, $R_L$, $alpha$ and $V_T$. (6%)
c) If $I_{EE} = 2mA$, what should be the value of $R_L$ to achieve $A_d = 40$? (4%)
d) Choose another set of $I_{EE}$ and $R_L$ to maintain $A_d = 40$ while reducing the power consumption by half compared to the situation in part c). (2%)
e) What is the maximum $A_d$ that the circuit can achieve if the DC levels at the output nodes $V_{op}$ and $V_{om}$ must be equal to or higher than 0V? (3%)

7. [20%] The circuit below is a BJT differential amplifier with $V{CC} = V{EE} = 3 V$. The DC current source $I{EE}$ is assumed ideal. The two transistors are identical, operate in the active region, and have $alpha = 0.99$ and $VA = infty$. The thermal voltage $VT$ is 26 mV at room temperature. The input is purely differential, i.e., $V{ip} = -V{im} = V{id}/2$.
a) Draw small-signal differential-mode half circuits of the differential amplifier. (5%)
b) Express the differential voltage gain Ad, which is defined as (Vop - Vom)/Vid, in terms of IEE, RL, alpha and VT. (6%)
c) If IEE = 2mA, what should be the value of RL to achieve Ad = 40? (4%)
d) Choose another set of IEE and RL to maintain Ad = 40 while reducing the power consumption by half compared to the situation in part c). (2%)
e) What is the maximum Ad that the circuit can achieve if the DC levels at the output nodes Vop and Vom must be equal to or higher than 0V? (3%)

Added by Charles L.

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