Output amplifier

DifTerential amplifier. An operational amplifier with two inputs of opposite-gain polarity with respect to its output. Differential-output amplifiers can also have two opposite-sense outputs. 

Figure 19. CCD array architecture, showing the parallel and serial transfers required to move the pixel charge to the output amplifier.

Carlson, 1988] Carlson, E. (1988). An output amplifier whose time has come. Hear-inglnstr., 39 30-32. 

The AC coupling of the output amplifier sets the lower cutoff frequency of the noise spectrum (time constant is AjCi). In our experiment, this frequency is below 1 Hz. 

Set up amplification of output phage as well as titration of input and output phage. Titer input, acid wash (if desired), and output. Amplify output or acid wash (if desired). 

A photomultiplier is thus a photocathode with the output amplified by an electron multiplier. The initial photocurrent can be amplified by a factor of 10 . Photomultipliers are thus useful when it is necessary to detect low intensities of light, as in stellar photometry, star and planet tracking in guidance systems, and more mundanely in process control. 

K. Because of the small pixel size (ISpmx 15 pm) and the buried-channel structure, saturation occurs at approximately 7 x 10 electrons per pixel. In addition to the large number of elements the array structure is fabricated such that noise-free summation of the charge from two or more pixels (prior to the output amplifier) can be carried out. This type of pixel summation should enhance its capability to perform low contrast, diffuse imaging. The device has line and point imperfection of less than 1 % of the pixels [8.103]. 

Photons are detected using a Bendix BX 754A Photon Counter tube with an S-20 photosensitive surface and a background count rate of 10 - 20 counts per second. For experiments done in air, the emission of photons was detected using an RCA 6217 (cooled with dry ice and acetone to reduce the noise level) with an analog output amplified with an electrometer whose output was fed to a strip chart recorder. ... 

Figure 20. Overview of the programmable decade amplifier module, consisting of a variable gain input amplifier, a 70-kHz filter, and an output amplifier with stepwise adjustable gain.

Each detector pixel transfers charge to a multiplexer cell that contains a complete readout circuit. The signal from this circuit is sent to the output amplifiers via shift registers. An early version of the circuit for the detection and transfer of charge deposited by the radiation is shown Fig. 6. 

FIGURE 6 An electrical schematic of the unit cell for each pixel along with the shift registers and output amplifier. 

The cosmetics of the detector are excellent except for one quarter which is not functioning, probably due to a failed output amplifier. Otherwise, only part of one line in one quarter is dead, together with a few isolated pixels. The low pixel defect rate arises from the monolithic silicon construction of the device. The noise is dominated by shot noise arising firom emission by the output amplifiers. About 27,000 electrons are deposited per pixel in the least affected parts, rising to 4 times this in the worst. Thus the effective read noise is about 160 electrons. The electronic read noise per se is estimated to be about 30 electrons. Some progress is possible towards reducing the amplifier-induced noise, for example by deareasing the readout amplifiers ON time which is about 6 seconds at present. 

The unit cell array is divided into four quadrants for parallel readout. At the edge of the quadrant, a set of FET switches are used to scan through each column to connect to the output bus corresponding to that quadrant. An output amplifier is at the end of the bus. The output amplifier is another soured follower. 

The output amplifiers are located near their bonding pads to reduce the effects of bias current generated photons in the output amplifiers, thus making it more difficult to couple those photons into the unit cell array. The output amplifiers have been extensively guard banded and covered with a metal layer to maximize their isolation. 

The basic blocks are the charge-sensitive loop, the pole-zero adjustment network, and the output amplifier. The charge-sensitive loop determines, to a major extent, the noise performance of the preamplifier. 

If the charge-sensitive loop has correct dc conditions, troubleshooting can continue by checking the output amplifier. 

Once this is fixed, you can go ahead by testing the dc condition of the output amplifier. Measure voltages at points E,... 

Point out that VE must be close to VF, and that VG must be near OV. Besides, the current flowing across R19 ( 5 mV) splits in almost equal parts between Q7 and Q8, about 2.5 mA in each transistor. 2.5 mA will then flow R9, thus keeping VH to about 17.7V. VL must be about. 7 V more positive. If the voltages VE, VF, VG, VH, VL differ considerably from the stated values, and especially if VG is positive or negative of some volts, then the output amplifier is defective. To troubleshoot it, first of all check the output stage and determine whether or not current flows across R24 and R26. The presence of current across R24 and R26 tells you that QIO and Qll are alive. If it is so, focus your attention on Q7, Q8, Q9. Disconnect Jumper AC-AB, in which case you open the feedback loop, and you can check the behaviour of Q7, Q8 in the open-loop situation. Disconnect Q9 and make sure that the currents across Q7, Q8 are not very different from each other, or at least that none of them are at zero. In this way you check the condition of Q7, Q8. If required, replace the defective component. Check then with the transistor tester Q9, and if necessary, replace it. Then, again connect Q9 and reintroduce the Jumper. The dc condition of the output amplifier should now be correct. 

The output signal from the photo-multiplier is fed through junction G to the amplifier section consisting of a buffer IC (Z203), a level discriminator 1C (Z202), and the output amplifier composed of Q201 at TPl on the emitter of Q204. 

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