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| Preamplifier
Short Form Catalog in PDF Format Click on any drawing on this page to see enlarged version. (Must have Adobe Reader to view these PDF drawings.) Download free version of Adobe Reader here  This section describes the amplifier circuits recommended for Judson detectors. The PA-5, PA-6, PA-7 and PA-9 series preamplifiers are current gain amplifiers recommended for photovoltaic detectors and applications. Voltage mode preamplifiers, for use with our photoconductive detectors, include the PA-101, PA-8200 and PA-300 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 current-mode 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 en(f) = [(en2/ZD2)
+ in2 + (4kT/RD) + (4kT/RF)]1/2
ZF This simplified noise equation provides a good approximation of the total voltage noise density (V/Hz1/2) 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 en divided by the detector reactance ZD, where ZD = RD/(1+ (2pf)2 CD2 RD2)1/2 • Preamplifier current noise in • Johnson thermal current noise from the detector shunt resistance RD • Johnson thermal current noise from the preamp feedback resistance RF The total current noise is then multiplied by the transimpedance gain ZF, where ZF = RF/(1+ (2pf)2 CF2 RF2)1/2 Analysis of the simplified noise equation shows the following: • In situations where ZD is large (>10Kohm) the preamplifier current noise in is more important than the voltage noise en. This is generally the case when using high-impedance detectors (InSb, cooled Ge, small-area Ge) at moderate frequencies. Choose a preamp with low in. • In situations where ZD is small (<1Kohm), the preamp voltage noise en becomes more important. This is generally true with low-impedance detectors (InAs, large-area Ge). Choose a preamp with low en. • Larger RF adds less current noise. For highest sensitivity, RF should be greater than RD when practical. Preamp Noise Figure: A general method for evaluating noise performance of a preamplifier is the noise figure, NF, which indicates what portion of the system noise is caused by the preamp. NF = 10 log10 [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 NF of 0.1 to 3 dB is considered satisfactory. Preamps with NF >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 Ib and input offset voltage Vos become important. In an ideal op-amp, Ib and Vos are zero. In reality they have non-zero values. Together with the detector RD they produce a "dark current" ID: ID = Ib + (Vos/RD) The DC offset voltage at the preamp output is equal to ID x RF. Ib and RD each have a non-linear dependence on temperature. The offset voltage at the preamplifier output will therefore drift with temperature changes. To minimize offsets and drifting: For high-impedance detectors, choose a preamp with low Ib. For low-impedance detectors, choose a preamp with low Vos. Consider stabilizing the detector temperature by using one of Judson's integral TE-cooler packages. The transimpedance (or current-mode) 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 op-amp 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).
The PA-9 preamplifier is ideal for high-frequency performance with high-impedance photovoltaics such as cryogenically cooled InSb and Ge. The PA-9 offers low current noise and ultra-low voltage noise. However, its relatively high DC offset voltage makes it less suitable for DC applications than other Judson preamps. The PA-9 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).
PA-5, PA-6, PA-7 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, lock-in amplifiers, or
A-to-D 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
multi-channel models) have switch-selectable gain. The PA-7 is an
excellent general purpose preamplifier for most high shunt resistance (RD
> 25Kohm) detectors, including small area J16 Series Ge and all J16TE2
Series cooled Ge. It has extremely low current noise and current offset.
For most applications, the PA-7-70 with high gain of 107 V/A
offers best performance and versatility. However, for applications where
107 V/A gain is unusable (due to bandwidth or DC saturation),
the PA-7-60 or PA-7-50 are suitable alternatives. The PA-6 is a general
purpose preamplifier recommended for intermediate shunt resistance
(400ohm<RD<50Kohm) detectors, including large area J16
Series room temperature Ge. The PA-6 has very low voltage noise and
offset voltage, which significantly reduces low-frequency noise and DC
drift. Standard gain settings are listed in the specification table
below; custom gain settings are available. The PA-5 is
recommended for low impedance detectors (RD<400ohm),
including J12 Series room temperature InAs and J12TE2 Series InAs. 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 105, 104,
and 103 V/A (switch-selectable). Custom gain settings are
available. 
Fig. 7
Fig. 9
The PA-12 is a two-stage preamplifier with selectable gains and internal reverse bias circuit for the photodiode. The first stage has three customer selectable gains and three selectable output coupling modes. The second stage is a non-inverting op-amp with three fixed gains with AC or DC output coupling modes.
PA-7:4C, PA-7:16C and PA-7:32C Multi-Channel Preamplifiers The PA-7:4C, PA-7:16C and PA-7:32C Series multi-channel preamplifiers are designed primarily for use with Judson's NIR Array Series and X-Y Sensors. The preamp gain is fixed as specified at the time of purchase. Standard gain settings are 107 or 106 V/A; others are available on a custom basis. While zero-volt bias is recommended for J16P Series arrays in most applications, the preamp is also available with an optional detector bias adjust. Biasing the photodiodes improves response time and high-power linearity, but also increases dark current.
Voltage Mode Preamplifiers may be used with photoconductive HgCdTe or with low-impedance 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. PA-101 HgCdTe Preamplifier (5 Hz - 1 MHz): The Model PA-101 low-noise 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 factory-selected for optimum detector performance. When ordering the preamp separately, please specify detector resistance and required bias current. The Model PA-101 may also be used without bias for J12 Series InAs.
PA-8200 PbS and PbSe Preamplifier: The Model PA-8200 low-noise voltage preamplifier is recommended for all J13 and J14 Series detectors. A load resistor is selected to match the detector resistance. Preamp gain and typical bandwidth specifications are listed in the table opposite. For best results, choose the preamp model with the narrowest suitable bandwidth to keep preamp noise to a minimum.
PA-300 HgCdTe Preamplifier (DC - 1.0 MHz): The Model PA-300 current preamplifier is designed for operation with J15D Series HgCdTe detectors. The PA-300 is designed using a bridge circuit on the front end of an operational amplifier to deliver constant bias voltage across the detector. The PA-300 is recommended for detectors used over a wide dynamic range in applications including FTIR's and laser monitoring. The PA-300 also has a first order linearity correction in the form of a positive feedback resistor.
Preamplifier Equivalent Circuits shown below.
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