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Understanding the FET Charge Amplifier Circuit: Dynamics of Voltage and Offset Effects

This report delves into the intricacies of the FET charge amplifier circuit. It explains the significance of the base voltage setting, the role of bypass capacitors in stabilizing voltage, and the impact of emitter followers on current amplification. The complexities introduced by folded cascode connections, offset voltage, and dynamic range are thoroughly examined. Additionally, the interactions between offset voltage and dynamic range are explored, illustrating how changes in these parameters affect output voltage and gain. Future investigations will target the relationship between offset voltage and dynamic range.

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Understanding the FET Charge Amplifier Circuit: Dynamics of Voltage and Offset Effects

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  1. Status Report 2010.7.9 Atsushi Nukariya

  2. Circuit of FET charge amplifier

  3. Explanation of Circuit ・○ ・・・ Setting base voltage of ○. ・○ ・・・ Cutting AC. → This capacitor stabilizes voltage which ○ sets. (Bypass capacitor)

  4. Explanation of Circuit ・○ ・・・ Emitter follower. → Amplification of current. Decreasing effects of previous circuit. Amplification of voltage doesn’t occur. ・○ ・・・ Folded cascode connection. → Amplification of voltage. Source of noise is here.

  5. Offset voltage and dynamic range ・Offset voltage (DC of ●) affects on dynamic range.

  6. Offset voltage and dynamic range Voffset = 0 Voffset = -a Dynamic range of plus side a GND Dynamic range of minus side ーInput of pulser ーOutput of pre-amp ・ If offset voltage is plus, dynamic range of plus side becomes small and that of minus side becomes big. ・ If offset voltage is minus, dynamic range of minus side becomes small and that of plus side becomes big.

  7. Input pulse ・ Pulser : Agilent 81110A ・ Delay : 0 [ps] ？ ・ Width : 100.0 [μs] ・ LeadE : 100 [ns]？ ・ TraiE : 50.0 [ns]？ ・ High : +0.0 [mV] ・ Low : variable ・ 50 [Ω] into 50.0 [Ω] Width High Low

  8. Offset voltage GND ●(51Ω resistance forward) Offset voltage ーInput of pulser ーOutput of pre-amp ーOutput of probe ・Offset voltage is -240 mV. 　→ Dynamic range of minus side will be smaller than that of plus side.

  9. Dynamic range Vin = 140 [mV] Vin = 660 [mV] Vin = 1650 [mV] ーInput of pulser ーOutput of pre-amp ・Dynamic range of minus side is -100mV. ・Dynamic range of plus side is 1420mV or more.

  10. Dynamic range Vin = 660 [mV] Vin = 1650 [mV] ーInput of pulser ーOutput of pre-amp ・Increasing time that output voltage of pre-amp is -100mV. 　→ Amplification of minus side becomes large. 　→ Time that output of pre-amp is minus after amplification of plus side becomes long.

  11. Problem in dynamic range ・ Dynamic range of plus side is unknown. 　→ 1420mV is the limit voltage that dynamic range of plus side can be obtained correctly when input pulse’s width is 100.0μs. 　→ Using the saw pulse instead of the rectangular pulse. ・ Dynamic range of minus side is smaller than Kato-san’s expectation value. 　→ I don’t understand Kato-san’s idea completely. Obtaining dynamic range of plus side and calculating of dynamic range are next plan. I wonder this result is caused for the frequency response.

  12. Gain ▲ Amplification of plus side ▲ Amplification of minus side Output voltage Input voltage ・Gain formula is (output voltage of pre-amp) / (input voltage) . 　→ Gain of plus side is below 1. Gain of minus side can’t be obtained because of exceeding from dynamic range. 　→ Kato-san said that this result is correct. Because if this result is correct I can’t explain the effect of amplification of voltage at folded cascode connection, I can’t understand this result.

  13. Next plan ・ Clarification of the relation between offset voltage and dynamic range. 　→ Change of shape of input pulse. ・ Understand of the role of folded cascode connection about gain.

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