Preamplifiers
Preamplifier Short Form Catalog in PDF Format
Preamplifiers
This section describes the amplifier circuits recommended for Judson detectors. The PA5, PA6, PA7 and PA9 series preamplifiers are current gain amplifiers recommended for photovoltaic detectors and applications. Voltage mode preamplifiers, for use with our photoconductive detectors, include the PA101 and PA8200 series preamps. Some general information on our preamplifiers follows:
Noise Sources: Figure 1 shows the various noise sources of the detector/preamp system. Values for the preamp noise sources en, in, Vos and ib are listed in the specification tables for each Judson currentmode preamplifier. The preamp noise sources, together with the detector characteristics, determine the system noise. While a complete analysis of detector system noise is beyond the scope of this guide, the effects of the various noise sources can be summarized by the following approximation:
Total e_{n}(f) = [(e_{n}^{2}/Z_{D}^{2}) + i_{n}^{2} + (4kT/R_{D}) + (4kT/R_{F})]^{1/2} Z_{F}
where k is Boltzmann's constant and T is temperature in degrees Kelvin.
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This simplified noise equation provides a good approximation of the total voltage noise density (V/Hz^{1/2}) at the preamplifier output. Note that the noise is dependent on the frequency f, and is normalized to a 1 Hz noise bandwidth. The four terms in the brackets represent the four main sources of current noise:
• Preamplifier noise voltage e_{n} divided by the detector reactance Z_{D}, where
Z_{D} = R_{D}/(1+ (2f)^{2} C_{D}^{2} R_{D}^{2})^{1/2}
• Preamplifier current noise i_{n}
• Johnson thermal current noise from the detector shunt resistance R_{D}
• Johnson thermal current noise from the preamp feedback resistance R_{F}
The total current noise is then multiplied by the transimpedance gain Z_{F}, where
Z_{F} = R_{F}/(1+ (2pf)^{2} C_{F}^{2} R_{F}^{2})^{1/2}
Analysis of the simplified noise equation shows the following:
• In situations where Z_{D} is large (>10Kohm) the preamplifier current noise i_{n} is more important than the voltage noise e_{n}. This is generally the case when using highimpedance detectors (InSb, cooled Ge, smallarea Ge) at moderate frequencies. Choose a preamp with low i_{n}.
• In situations where Z_{D} is small (<1Kohm), the preamp voltage noise e_{n} becomes more important. This is generally true with lowimpedance detectors (InAs, largearea Ge). Choose a preamp with low e_{n}.
• Larger R_{F} adds less current noise. For highest sensitivity, R_{F} should be greater than R_{D} when practical.
Preamp Noise Figure: A general method for evaluating noise performance of a preamplifier is the noise figure, N_{F}, which indicates what portion of the system noise is caused by the preamp.
N_{F} = 10 log_{10} [Total Noise / Detector Noise]
A perfect preamplifier has a Noise Factor of 0 dB, indicating that the preamp noise contribution is negligible compared to the detector noise. A N_{F} of 0.1 to 3 dB is considered satisfactory. Preamps with N_{F} >3 dB add significant noise to the system. See Fig. 6 for noise figures of Judson transimpedance preamplifiers at 1 KHz.
DC Applications  Offset Drifting: In DC applications, the preamp input bias current I_{b} and input offset voltage V_{os}become important. In an ideal opamp, I_{b} and V_{os} are zero. In reality they have nonzero values. Together with the detector R_{D} they produce a "dark current" I_{D}:
I_{D} = I_{b} + (V_{os}/R_{D})
The DC offset voltage at the preamp output is equal to I_{D} x R_{F}. I_{b} and R_{D} each have a nonlinear dependence on temperature. The offset voltage at the preamplifier output will therefore drift with temperature changes. To minimize offsets and drifting:
For highimpedance detectors, choose a preamp with low Ib. For lowimpedance detectors, choose a preamp with low V_{os}. Consider stabilizing the detector temperature by using one of Judson's integral TEcooler packages. The transimpedance (or currentmode) preamplifier circuit of Fig. 1 is recommended for most PV detector applications, for frequencies up to 1 MHz. It offers lowest noise and best linearity under a wide range of conditions. The characteristics of the opamp circuit maintain the diode near 0V bias. All the photocurrent from the detector essentially flows through the feedback resistor RF. The feedback capacitance CF is added to control gain peaking (Fig. 2). The value of CF depends on the detector capacitance. It is installed at the factory to provide stable preamplifier performance with a particular detector model. The values of RF and CF, together with the detector characteristics RD and CD, determine the overall frequency response of the system (Figs. 3, 4, 7, 8).
PA9 Preamplifier
The PA9 preamplifier is ideal for highfrequency performance with highimpedance photovoltaics such as cryogenically cooled InSb and Ge. The PA9 offers low current noise and ultralow voltage noise. However, its relatively high DC offset voltage makes it less suitable for DC applications than other Judson preamps. The PA9 has fixed gain. When ordered with a detector, the preamp is matched to the detector for maximum gain and sensitivity. Alternatively, the customer may specify gain or minimum required bandwidth. Bandwidth is a function of detector resistance and capacitance as well as preamp gain (Figs. 3 and 4).
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Model  1st Stage Gain (V/A)  1st Stage Bandwidth (Maximum) 
PA970  107  DC to 100KHz 
PA960  106  DC to 300KHz 
PA950  105  DC to 750KHz 
PA944  2.5x104  DC to 1MHz 
Typical Specifications Model PA9 Preamplifiers
2nd Stage Gain  20  dB 
Voltage Noise Density @1KHz  6.5  nV Hz1/2 
Voltage Noise from 0.1 to 10 Hz  1.0  µVpp 
Current Noise Density @ 1KHz, 1E7 Gain †  0.04  pA Hz1/2 
Input Offset Voltage  ± 10  mV 
Input Bias Curren  t  ± 1 pA  
Maximum Output  1st stage = 6 2nd stage = 10  Vp 
p 
ance  < 50  W 
Power Requirements  ±12 or ±15 20  VDC mA 
PA5, PA6, PA7 Preamplifiers
Current Mode Preamplifiers convert the current output of a photovoltaic Ge, InAs, or InSb detector into a voltage output. They amplify the signal for subsequent use with oscilloscopes, lockin amplifiers, or AtoD converters. Three different preamp models each offer specific advantages, depending on detector type and bandwidth requirements. A comparison of preamp noise figure as a function of detector reactance is graphed in Fig. 6. All units (except multichannel models) have switchselectable gain.
The PA7 is an excellent general purpose preamplifier for most high shunt resistance (R_{D} > 25Kohm) detectors, including small area Ge and TECooled InGaAs, Ge, InAs, SWIR PV MCT, and LN2 Cooled InSb. It has extremely low current noise and current offset. For most applications, the PA770 with high gain of 10^{7} V/A offers best performance and versatility. However, for applications where 10^{7} V/A gain is unusable (due to bandwidth or DC saturation), the PA760 or PA750 are suitable alternatives.
The PA6 is a general purpose preamplifier recommended for intermediate shunt resistance (400ohm<R_{D}<50Kohm) detectors, including large area Ge and TECooled InGaAs, Ge, InAs, MWIR PV MCT, and LN2 Cooled InSb. J16 Series room temperature Ge. The PA6 has very low voltage noise and offset voltage, which significantly reduces lowfrequency noise and DC drift. Standard gain settings are listed in the specification table below; custom gain settings are available.
The PA5 is recommended for low impedance detectors (R_{D}<400ohm), including InAs and MWIR PV MCT. It has extremely low voltage noise and low voltage offset. However, its high current noise and current offset make it unsuitable for detectors with high impedance. Standard gain is 10^{5}, 10^{4}, and 10^{3} V/A (switchselectable). Custom gain settings are available.
5
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Model  PA7 Series  PA6 Series  PA5  Units 
PA770   0  PA750  PA660  PA650  PA550 
Transimpedance : (Switch Selected)  High Med Low  1E7 1E6 1E5  1E6 1E5 2.5E4  1E5 4 1E4  1E6 E5 2.5E4  1E5 2.5E4 1E4  1E5 4 1E3  V/A 
Bandwidth  nF  @ High Gain @ Med Gain @ Low Gain  8 60 150  60 1  50 200  150 200 200  60 150 200  150 200 200  200 200 200  KHz 
Input Offset Voltage (Vos) Input Bias Current (ib) Voltage Noise Density (en) @1KHz Voltage Noise from 0.1 to 10Hz Current Noise Density (in)@1KHz†  ±250 ±0.001 12 1.5 .04  ±250 ±0.001 12 1.5 .13  ±250 ±0.001 12 1.5 .04  ±100 ±12 4.5 .080 .5  ±100 12 4.5 .080 .64  ±80 ±30 1.1 .035  1  V nA nV Hz^{1} 
^{/2} µVp 
p Hz^{1/2} 
e Maximum Output Voltage Power Requir  ements  < 100 ± 10 +12V and 12VDC @ 10mA  W Vpp 
Recommended for Detector Series:  J16, J16TE1, J16TE2, J16D, J10D  J16, J12TE2, J12TE3  J12, J12TE2  
Model  # of Channels  Gain (V/A)  Bandwidth (Max) 
C70  4  1E7  DC to 10KHz 
PA7:4C60  4  1E7  DC to 60KHz 
(Vos)  ±200  µV 
Input Bias Current (ib)  ±40  pA 
Voltage Noise Density (en) @1KHz  18nVHz^{1/2} 
Voltage Noise from 0.1 to 10  Hz  2  µVpp 
Current Noise Density† in @ 1KHz  AHz^{1/2} 
Output Impedance  < 100  W 
Maximum Output Voltage  ±10  Vpp 
Power Requirements PA7:4C (4 channel)  ±15 @ 40  VDC ma 
Use with Detector Series:  NIR Arrays 
† At Gain = 1E7 V/A. Lower gains increase Current Noise Density. 
Voltage Mode Preamplifiers
Voltage Mode Preamplifiers may be used with photoconductive HgCdTe or with lowimpedance photovoltaics such as InAs. With photoconductive detectors, a constant bias current or constant bias voltage is applied across the detector element. The element changes resistance in response to incident photons, and the resulting change in voltage is amplified by the preamp. A blocking capacitor or DC offset circuit is required to block the constant DC bias. With photovoltaic detectors, the photocurrent generated in the detector induces a voltage across the preamp input impedance. This voltage is amplified. A lower input impedance generally results in faster frequency response, but also adds more noise to the system
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PA101 HgCdTe Preamplifier (5 Hz  1 MHz): The Model PA101 lownoise voltage preamplifier is recommended for all J15 Series HgCdTe detectors. An external bias resistor is used to set the constant bias current required for PC detector operation. When purchased with a detector, the preamp includes a bias resistor factoryselected for optimum detector performance. When ordering the preamp separately, please specify detector resistance and required bias current.
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Preamplifier Equivalent Circuits shown below.
Model  P/N  Gain  Bandwidth (Hz)  Input Noise Voltage (nV Hz^{1}  ^{/2})  Input Impedance (ohms)  Max. Output (Load >1Kohm)  p)  Detector Bias  Power Requirement  Case Dimensions (Excluding Connectors) 
(VDC)  (mA) 
PA101  4  90113  10  10Hz to 1MHz  1.5  10K  1  0  Bui  ltin  ±15  200  4.125" x 2.5" x 1.75" 
           4.125" x 2.5" x 1.75" 










