- Sep 21, 2017 -
An amplifier is a device that amplifies the voltage or power of an input signal, consisting of a tube or transistor, a power transformer, and other electrical components. Used in communications, broadcasting, radar, television, automatic control and other devices.
Operational amplifiers are one of the most common and most important elements in analog-to-digital conversion circuits. Fully differential op amp refers to the input and output are differential signal op amps, compared with the ordinary single-ended output op amp has the following advantages: the output voltage swing is greater; better inhibition of common mode noise; Low noise; suppression of harmonic distortion of the even order is better. So usually high-performance op amps are mostly fully differential. In recent years, the differential differential op amp's higher unit gain bandwidth and greater output swing make it more widely used in high-speed and low-voltage circuits. With the increasing data conversion rate, high-speed analog-to-digital converters are becoming increasingly demanding, and high-speed analog-to-digital converters require high gain and high unity gain bandwidth op amps to meet system accuracy and fast build needs. Speed and accuracy are two of the most important performance indicators of analog circuits. However, the requirements of both are constrained and contradictory. So it is difficult to meet these two requirements at the same time. Folded cascode technology can solve this problem more successfully. The op amp of this structure has high open loop gain and high unit gain bandwidth. The disadvantage of the fully differential op amp is that the common-mode loop gain of the external feedback loop is very small and the output common-mode level can not be accurately determined. Therefore, it is common to add the common-mode feedback circuit.
The choice of op amp structure
There are three types of operational amplifiers: (a) a simple two-stage op amp, (b) a cascaded cascode, and (c) a cascode, as shown in the previous stage of Fig. The design of the operational amplifier design requirements of the differential output amplitude of ± 4V, that is, all the output of the NMOS tube VDSAT, N and less than 0.5V, the output of all PMOS tube VDSAT, P and must also be less than 0.5V.
Main op amp structure
The op amp has two levels: (1) Cascode level increases DC gain (M1-M8); (2), common source amplifier (M9-M12).
Common mode negative feedback
For full differential op amps, in order to stabilize the output common-mode voltage, the common-mode negative feedback circuit should be added. In designing a balanced balanced fully operational op amp, the following should be taken into account: Common-mode negative feedback open-loop DC gain requirements are large enough, preferably in differential open-loop DC gain equivalent; common-mode negative feedback unity gain The bandwidth is also required to be large enough, preferably close to the differential unity gain bandwidth; in order to ensure the stability of the common mode negative feedback, under normal circumstances require common mode loop compensation; common mode signal monitor requires a very good linear characteristics; Independent of the differential mode signal, even if the differential mode signal path is off.
The op amp uses a continuous-time mode to achieve common-mode negative feedback.
This structure shares the current mirror and output load in the input stage of the common mode amplifier and differential mode amplifier. This, on the one hand, reduces power consumption; on the other hand, it ensures that the common-mode amplifier and the differential-mode amplifier are consistent in the AC characteristics. Because the output stage of the common-mode amplifier and the output stage of the differential-mode amplifier can be fully shared, the capacitor compensation circuit is the same. As long as the differential mode amplifier frequency characteristics are stable, the common mode negative feedback is also stable. This common-mode negative feedback circuit allows the fully differential operational amplifier to be designed like a single-ended output op amp, regardless of the effect of the common-mode negative feedback circuit on the fully differential amplifier.
Voltage bias circuit: wide swing current
Three voltage biases are required in the cascode input stage, in order to make the dynamic range of the input stage larger and the wide swing current source to produce the three required bias voltages.