Digital Signal Processing Reference
In-Depth Information
Gain [dB]
output referred IP3 [dBm @50
Ω]
Low-frequency cut-off
caused by lock-in of
30
30
offset compensation.
25
25
Steep decrease at high-
frequency cut-off due
20
20
to 8 coinciding poles.
15
15
OIP
3
measured using IM
3
intermodulation test with
1MHz tone separation.
10
10
2.10
7
10
8
10
9
Frequency [MHz]
Figure 7.10.
Measurement result of the eight-stage open-loop amplifier. The
output-referred third-order intermodulation intercept point (OIP
3
)
was calculated using a two-tone intermodulation test with 1 MHz
separated frequencies. The OIP
3
was the obtained using OIP
3
=
P
out
−
1
2
IM
3
.
found in Figure 7.8. The proposed nonlinear loaded open-loop amplifier out-
performs comparable structures reported in the literature [Lee06, Duo06].
7.7
Overview and future work
The nonlinear characteristics of an active element can be improved by em-
bedding this element in the forward path of a feedback system: a large loop
gain makes that the transfer function of a closed-loop system is determined by
the characteristic of the feedback circuitry,
8
rather than the non-ideal transfer
function of the active element. What is often overlooked, though, is the fact
that distortion suppression only starts when some excess loop gain becomes
available to the system (Appendix A.2). Only starting from below the closed-
loop cut-off frequency, distortion is increasingly suppressed, down to the first
pole of the open-loop system.
This finding may lead to different conclusions, depending on the specific ap-
plication conditions of the feedback amplifier. If the amplifier is going to be
used in a low-speed high-accuracy circuit such as a sensor or an audio pream-
plifier, the low-frequency gain of the amplifier must be as high as possible,
while the 3 dB cut-off frequency of the
open-loop
system must be of the same
order of magnitude as the bandwidth that is required by the application. In
8
Because of their decent linearity performance, passive devices are commonly used in the feedback path.
