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I'm having some difficulty understanding the frequency response of a charge mode amplifier, circuit shown in Figure 1 below.

                        Figure 1: Ideal charge mode amplifier circuit (1)

I believe I understand the principles behind an charge mode amplifier circuit, however I'm rather confused about the frequency response. Considering Figure 1 above, my understanding is that R1 and Cf will act as a low pass filter, where frequencies above a specific cutoff frequency (related to values of R1 and Cf) will be attenuated. However, having had a read through a Texas Instruments (TI) PDF (2) discussing this type of circuit, the frequency response doesn't appear to follow this rule (I was expecting a low pass filter response). The circuit and frequency response described by TI is shown in Figure 2 below.

            Figure 2: Practical charge mode amplifier and frequency response (2)

I'd really appreciate if someone could help me understand this.

References

(1): https://commons.wikimedia.org/w/index.php?curid=16867828

(2): https://www.ti.com/lit/an/sloa033a/sloa033a.pdf

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  • \$\begingroup\$ Those are two different circuits. The first has one pole in its transfer function, the other 2 plus a pole at \$\omega=0\$. First: \$i_1=-i_F => \frac{V_{in}}{R_i}=-V_o.j\omega C\$ so \$\frac{V_o}{V_{in}}=-\frac{1}{j \omega R_i C} \$: A pole at \$\omega=0\$. \$\endgroup\$ – HarryH Jun 9 '18 at 18:56
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Your first circuit is an integrator. It ideally puts the pole at zero, so there's no "corner frequency" as such...it's just 20dB/decade with (ideally) infinite gain at DC.

The second schematic adds Rf, which plays against Cf to give you a low-pass pole. I think they switched the high-pass and low-pass formulae on the frequency response chart.

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