Bias circuit

The detector output signal is generated by a current preamplifier for photovoltaic detectors, such as InSb, and by a simple detector bias circuit shown in Fig. 4 for photoconductive detectors, such as PbS and Hg Ge. The voltage signal derived from the bias circuit is normally preamplified and forwarded to a phase-sensitive synchronous detector usually embodied in a lock-in amplifier (Stewart, 1970 Blass, 1976b). 

The behavior of the bias circuit of Fig. 4 is of some interest. Let (RD)0 be the equivalent detector resistance at zero signal. The quantity RL is the load resistance (preferably wire-wound or metal film see Stewart, 1970) and V0 is the background detector voltage that corresponds to a detector resistance Rd = (Rd)0. Then the output signal of the bias circuit, Fsig(a), is given by... 

Fig. 4 Simple photoconductive detector bias circuit. RD is the detector resistance, (/ D)0 the equivalent detector resistance at zero signal (i.e., background flux only on detector). RL is the load resistance, V0 the signal at (RD)0, Vsig the signal voltage at Rd.V0= VbJRD)(J[RL + (RJ0l Vsie(t) = Vba,RDl(RL + rd)> and Fba. is the battery voltage.

Figure 10J5 The first stage of a To next stage charge-sensitive preamplifier dc-coupled to the bias circuit.

This complex relationship means that if the cavity is held at resonance and the spectral line is swept, e.g. by Stark or Zeeman modulation, although other modulation schemes are possible, the cavity impedance changes and the reflected power incident on the Gunn device changes in sympathy with the spectral scan. This causes a current to flow in the Gunn oscillator circuit related to the spectral absorption profile, and therefore to its amplitude and area. That current can be readily transformer-coupled out of the Gunn bias circuit and detected S)mchro-nously with the modulation frequency. 

Integrated active bias circuit Single positive supply voltage Adjustable current from 1 to 10 mA 2dB noise figure at 900 MHz 25 dB gain at 100 MHz. 

FIGURE 6.8 Grounded grid equivalent circuits (a) low-frequency operation, (b) microwave frequency operation. The cathode-heating and grid-bias circuits are not shown. 

Figure 5.12.3 shows circuits most commonly used with photon detectors. The bias circuit illustrated in (a) is well-suited for photoconductors. The signal voltage is measured across a load resistor. The simple circuit in (b) is appropriate for a PV detector without an external bias. The more complex circuit in (c) is for a reverse-biased PV detector. In this circuit, which resembles (a) to some extent, an external voltage is applied across the detector and a load resistor arranged in series. 

It is generally most convenient to work with noise currents. For a typical PC bias circuit (see Section 4.10), the load resistor and the detector resistor are in parallel across the output. In that case, the net noise current is the RSS of the two noise currents ... 

The /- Vcurves (expressed either as a graph or an equation) also relate the detector current to its voltage The interaction between the detector and the bias circuit is most easily understood by examining the /- Vcurve and the load line. Two I- Vcurves are shown in each figure - one for some arbitrary irradiance the other is for a slightly different irradiance. 

There is no fundamental reason why the bias circuit described for a PC detector cannot be used with a PV detector. (The bias battery would have a very low voltage, or even zero.) In practice, this is never done because a much more convenient scheme is available. 

Derivations Figure 4.14 is helpful in deriving those relationships. It shows two I-Vcurves for a hypothetical detector one for a photon irradiance and another where the arrival rate has increased by the photon signal irradiance Q. It also shows load lines for three bias circuits. The line labeled actual load line represents a realistic circuit. The other two represent idealizations, one for the short-circuit limit (a load resistor much less than the detector resistance vertical load line), and the other for the open-circuit limit (load resistance much greater than the detector resistance horizontal load line). 

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