The preamps are integrating circuits that convert the image charges produced by the calorimeter cells to voltages that are proportional to the input charge. The upgrade consists of three stages, each of which is essentially a fast inverting operational amplifier. Only the first stage was needed earlier because it was not necessary to drive a fast pulse into the resistive cable. Since the preamp output is also back terminated, a large current with twice the output voltage is required. The second and third stages were added so that the first stage's performance would not be compromised.
The first stage needs to accomplish a number of tasks while meeting certain requirements. It acts as a charge sensitive amplifier by using a capacitor in feedback to integrate the incoming current pulse to produce a voltage proportional to the integrated charge.
By studying the pattern of detector capacitance, we have classified all the calorimeter cells into 14 categories. We will build 14 species of preamps geared to compensate for the detector capacitance in the electromagnetic layers, haydronic layers, and the central calorimeter and end-cap calorimeter cells.
The input impedance of the preamp is achieved by making the gain of the first stage the appropriate function of frequency. For a perfectly integrating preamp, the feedback element through which the input current flows is a capacitor. For compensated preamps, a resistor of the appropriate value is added in series with the load capacitor to tune the open loop gain for the feedback RC in order to maintain a purely resistive input impedance. Power is delivered across low and high frequency capacitors through resistors on each preamp to provide smoothing. The capacitor presents a small impedance at the pulse shaping frequencies while presenting a sufficiently large impedance to prevent saturation of the output pull-down.
