CLC425 datasheet(7/9 Pages) NSC

7 / 9 page

Inverting Gains Less Than 10V/V

The lag compensation of Figure 9 will achieve stability

for lower gains. Placing the network between the two

input terminals does not affect the closed-loop nor noise

gain, but is best used for the invering configuration

because of its affect on the non-inverting input imped-

ance.

Single-Supply Operation

The CLC425 can be operated with single power supply

as shown iin Figure 10. Both the input and output are

capacitively coupled to set the dc operating point.

Low Noise Transimpedance Amplifier

The circuit of Figure 11 implements a low-noise transim-

pedance amplifier commonly used with photo-diodes.

The transimpedance gain is set by Rf. The simulated

frequency response is shown in Figure 12 and shows

the influence Cf has over gain flatness. Equation 4

provides the total input current noise density (ini) equa-

tion for the basic transimpedance configuration and is

plotted against feedback resistance (Rf) showing all

contributing noise sources in Figure 13. This plot indi-

cates the expected total equivalent input current noise

density (ini) for a given feedback resistance (Rf). The total

equivalent output voltage noise density (eno) is ini

∗R

Very Low Noise Figure Amplifier

The circuit of Figure 14 implements a very low Noise

Figure amplifier using a step-up transformer combined

with a CLC425 and a CLC404. The circuit is configured

with a gain of 35.6dB. The circuit achieves measured

Noise Figures of less than 2.5dB in the 10-40MHz

region. 3rd order intercepts exceed +30dBm for frequen-

cies less than 40MHz and gain flatness of 0.5dB is measured

in the 1-50MHz pass bands. Application Note OA-14 provides

greater detail on these low Noise Figure techniques.

CLC425

+Vcc

Figure 11: Transimpedance Amplifier Configuration

vin

=− ∗

Rf = 124

Ω

Rs = 75

Ω

Rg = 124

Ω

CLC425

Rin

Cin

39pF

Cf = 10pF

Icc=5mA

Ω

Figure 8: External Shunt Compensation

Figure 12: Transimpedance Amplifier Frequency Response

CLC425

Vout

Rout

Vin

http://www.national.com

Figure 9: External Lag Compensation

Rout

Vac

Vcc

CLC425

Vcc

+ AvVac

Vout =

Figure 10: Single Supply Operation

Equation 4: Total Equivalent Input Refered Current









Figure 13: Current Noise Density vs. Feedback Resistance

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