# Driving the future of technology : Original solution in measurement and control

## Lecture for measurement

2019/9/23 posted

### Current amplifier (Transimpedance amplifier)

Contents List

#### Features of NF Current amplifiers

The features of NF Current amplifier, which is useful for detecting a very-small-current signal, are introduced below.

#### 1. Achieving frequency characteristics surpassing those of other manufactures' product

A general current amplifier consists of an op-amp and a feedback resistor.

A basic circuit is shown in Figure 1. The gain of this circuit is determined by feedback resistor Rf. And the upper cutoff frequency is determined by feedback capacitor Cf and Rf.
Since it is determined by Cf and Rf, the upper cutoff frequency of current amplifier cannot be increased even using an operational amplifier with a large gain bandwidth.

Figure 1: Current amplifier using feedback method

In order to expand the bandwidth, it is necessary to reduce Cf capacitance. However, in general, it is difficult to make Cf smaller than 0.1 pF due to the parasitic capacitance of components and circuits.
If the feedback resistance Rf is 10 GΩ and the feedback capacitance due to parasitic capacitance is 0.1 pF, the upper cutoff frequency is 160 Hz at maximum. At 0.1pF feedback capacitance, the relationship between the feedback resistance (Rf) and the upper cutoff frequency is shown in Table 1.

Table 1: Limits of upper cutoff frequency due to feedback capacitance

Feedback resistance Rf (Ω) Feedback capacitance Cf (F) Upper cutoff frequency (Hz)
10M 0.1p 160k
100M 0.1p 16k
1G 0.1p 1.6k
10G 0.1p 160

On our SA-600/CA-550/CA-650 series, by devising and adjusting circuit configurations and components mounting method, it is realized to reduce equivalent feedback capacitance due to parasitic capacitance to less than 0.1 pF and achieve increasing upper cutoff frequency.
The relationship of our SA-600/CA-550/CA-650 series between upper cutoff frequency, feedback resistor (gain) and equivalent feedback capacitance are listed in Table 2.

Table 2: Upper cut-off frequency, feedback resistance, and equivalent feedback capacitance of NF SA-600/CA-550/CA-650 series

Model name Upper cutoff frequency fc (Hz) Feedback resistance Rf (Ω) Equivalent feedback capacitance Cf (F)
SA-604F2/CA-554F2/CA-654F2 500k 10M 32f
SA-605F2/CA-555F2/CA-655F2 250k 100M 6.4f
SA-606F2/CA-556F2/CA-656F2 100k 1G 1.6f
SA-607F2/CA-557F2/CA-657F2 20k 10G 0.8f

Equivalent feedback capacitance:
Feedback capacitance calculated from upper cutoff frequency and feedback resistance.

The comparison of upper cutoff frequency between our SA-600/CA-550/CA-650 series and other manufactures is shown in Table 3. As clear from Table 3, these our products achieves wide bandwidth characteristics exceeding those of other manufactures.

Table 3: Comparison of upper cutoff frequencies

Gain (V/A) NF's SA-600/CA-550/CA-650 Series (Hz) Company A Constant gain model (Hz) Company C Variable gain model (Hz)
10M 500k 400k 23k
100M 250k 40k 8.8k
1G 100k 4k 3.5k
10G 20k 200 1.4k

*Investigation on June 16, 2017

Figure 2 and figure 3 show pulse-response waveforms when a square-wave with an input current of ± 5 nA. In the figures, NF's SA-607F2 is compared to company C's current amplifier. Both gains are 10 GV/A. In the case of the 1-kHz pulse shown in Figure 2, the rise time of the SA-607F2 is 20-times faster than that of company C's current amplifier. Moreover, company C's current amplifier cannot amplify the 10-kHz signal; However, the signal output through SA-607F2 is clearly shown.

Figure 2: Pulse-response waveforms at f=1 kHz

Figure 3: Pulse-response waveforms at f=10 kHz

*Investigation on June 16, 2017

Rise time

Company name Product name Rise time
NF SA-607F2 13µs
Company C Current amplifier 260µs

Related Product

Wideband Current Amplifier SA-600 Series

World's top level current amplifier with extremely low noise

Wide current range measurement (sub pico-Amp. to 1 µA)
•  Realize both high gain and wide bandwidth
•  Low noise current density (fA/√Hz order)

Wideband Current Amplifier CA-550 Series / CA-650 Series

CA-550 series and CA-650 series are low noise over wideband current amplifiers with a high gain and wide bandwidth.

Related keywords :  Ultra low noise amplifier, LNA

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