The features of NF Current amplifier, which is useful for detecting a very-small-current signal, are introduced below.
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.
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.
|Feedback resistance Rf (Ω)||Feedback capacitance Cf (F)||Upper cutoff frequency (Hz)|
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.
|Model name||Upper cutoff frequency fc (Hz)||Feedback resistance Rf (Ω)||Equivalent feedback capacitance Cf (F)|
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.
|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)|
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.
|Company name||Product name||Rise time|
|Company C||Current amplifier||260µs|
Related keywords : Ultra low noise amplifier, LNA