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(a) Figure Q1 shows an amplifier which is used to amplify the output current (dour) Cb) of a fuel cell breathalyzer, which may be regarded as a perfect current source g , (infinite impedance)

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(a) Figure Q1 shows an amplifier which is used to amplify the output current (dour) Cb) of a fuel cell breathalyzer, which may be regarded as a perfect current source g , (infinite impedance). Re-draw the circuit to show all sources of DC error. [12]

(b) | Hence derive an expression for the minimum current (referred to input) which Viot can be measured by the circuit, i.e. the input current which is equal to the total.. ( out 2 error. For minimum current error, should Rr be as large as possible, or as small

: Rx as possible? Choose a value for Rx and state the reasons for your choice.

 (c) Define a figure of merit for the operational amplifier, A1.

 (d) The breathalyzer produces a current of 1nA for every Ha wer in the blood alcohol concentration (BAC) in the breath sample (output current is linearly, proportional to change in BAC). If the breathalyzer is required to be accurate to By Deruune ( [ = P 0.052/L of BAC, what is the total error current which can be tolerated? [4]

(e) Rf a resistor in the range of 1? to 850k?. Al is one of the following three~ opamps listed in Table Q1 below. Choose a value for Rrand determine whichis the best amplifier for this application and what is the minimum BAC which could be detected using it?

Amplifier	Offset voltage	Bias current	Offset current
AD8675	10	500	100
OPA627B	40	1	0.5
LT1097CN8	50	250	250