Biased amplifier

Biased amplifier Shaped linear pulse Linear pulse proportional to amplitude of input pulse that lies above input bias level... 

Figure 1. Block diagram of single-photon time-correlation apparatus from Barker and Weston 11 HV, high-voltage supplies L, lamp PI, photomultiplier M, monochromator FURN, furnace C, sample cell LP, light pipe F, interference filter P2, photomultiplier AMP, amplifier DISCI, discriminator D1SC2, discriminator T-S, timer scaler DL, delay line TAC, time-to-amplitude converter BA, biased amplifier MCPHA, multichannel pulse-height analyzer TTY, teletype printer and paper-tape punch REC, strip-chart recorder.

The crystal must be cooled during data collection (i.e., if the high voltage is applied), by liquid nitrogen or by an electrical cooler. The biased amplifier (see section 4) allows expansion... 

The TAC output voltage is sent through a Biased Amplifier , AMP. The amplifier has a variable gain and a variable offset. It is used to seleet a smaller time window within the full-seale eonversion range of the TAC. 

Because of the high speed of the biased amplifier and the ADC in a TCSPC board, the photon pulses delivered to the ADC need not be broader than 50 to 100 ns. Therefore an extremely high preamplifier gain is not required, and AC coupling can be avoided. The setup can therefore be used up to a count rate of several 10 pulses per second. Examples for pulse height distributions recorded this way are shown in Fig. 6.13, page 227. 

Classie NIM-based TCSPC systems required caleulating the time scale from the (possibly not accurately known) TAC and MCA slopes and the gain of the biased amplifier. 

Coincidence and Dead-time Losses in y-Spectrometry. The influence of electronic effects at high-count rates on the performance of Ge(Li) detectors is considerable. The resolution of a detector can be degraded by effects within the amplification system, but these can be minimized by (i) the use of pole-zero cancellation, to prevent the pulse-height error caused by the tail of a preceding pulse and (n) baseline (or D.C.) restoration facilities to prevent similar errors caused by shifts in the apparent pulse baselines. The latter are a result of capacitative effects between the various stages of the overall amplifier, biased amplifier, and multi-channel analyser system. These effects can degrade the resolution of the detector but should not change the y-ray peak area. 

However, at high-count rate the probability of the loss of counts from a y-ray peak owing to the coincidence between two y-ray pulses detected at the same (or nearly the same) instant becomes important and has a significant effect on the accuracy of the measurement. Such losses cannot be accounted for by dead-time correction. It is important to realise that the coincidence loss depends not on the count rate at the multi-channel analyser, which might be low if a biased amplifier is used to select a region of interest, but on the total y-ray count rate at the detector. Since coincidence losses are rate dependent, samples and standards should be of comparable intensity or errors will result. 

A transistor, or n-p-n junction, is built up of two n-type regions of Si separated by a thin layer of weakly p-type (Fig. e). When the emitter is biased by a small voltage in the forward direction and the collector by a larger voltage in the reverse direction, this device acts as a triode amplifier. The relevant energy level diagram is shown schematically in Fig. f... 

The resistivity of the silicon is increased by making the whole detector a semiconductor p-i-n junction which is reverse biased by a potential applied to a thin film of gold on the outer faces. The silicon is doped with a small concentration of lithium, and the whole detector is cooled to liquid nitrogen temperature (77 K). The current which passes between the (gold) electrodes is now very small until an X-ray enters the detector, and the resultant current pulse can be amplified and measured. 

To make the entire eleetronic response linear with respect to tunneling gap s, a logarithmic amplifier is attached at the output of the current amplifier. A logarithmic amplifier can be made from a feedback amplifier, by replacing the feedback resistor with a diode, as shown in Fig. 11.4. The current-voltage characteristics of a good-quality, forward-biased silicon diode follow an exponential law over more than five orders of magnitude ... 

There are two ways to look at this circuit One is that this is a self-biasing circuit for a BJT amplifier. If this were an amplifier, the load would most likely be a resistor. (The term "self-bias means that the goal of the circuit is to make the collector current independent of device parameters such as Hfe and Vbe.). The second way to look at the circuit is that as far as the load is concerned, Ql, Rl, R2, and R3 form a current source that is, the current through the load is determined by Ql. Rl, R2, and R3. If this circuit were designed as a current source for the load, we could view the circuit as ... 

Capacitor C2 is necessary to preserve the bias of the amplifier. Remember that V3 is an AC source. For biasing, all AC sources are set to zero. Since V3 is a voltage source, it would be replaced by a short. Without C2, the source terminal of the jFET would be grounded when calculating the bias. This would destroy the bias and render the impedance measurement invalid. 

Figure 9.8. A total power radiometer with an AC-biased bolometer feeding an audio amplifier followed by a lockin amphfier. This design is used in BOOMERanG and will be used in the Planck HFI.

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