Next generation wireless networks, such as fifth generation (5G) networks require increased capacity. This increased capacity can be provided by supporting a large communication bandwidth. However, increasing the bandwidth of a wireless receiver is challenging, affecting not only radio frequency circuits but also requiring circuits traditionally operating at lower frequencies to operate at higher frequencies. Therefore, there is a requirement for new baseband circuits capable of supporting a large bandwidth.
In recent decades, baseband low impedance filtering stages have become key blocks for direct down conversion receivers. A typical solution employs a transimpedance amplifier (TIA) based on a differential amplifier with a feedback resistor-capacitor (RC) network, providing low input impedance at a virtual ground, and a first order filter response.
However, a TIA has several disadvantages. Designing a TIA with a large bandwidth, in particular exceeding 500 MHz requires a very high gain-bandwidth (GBW) amplifier. An operational amplifier (OPAMP) used to implement a TIA can achieve very high gain but not at very high frequencies. The stability and frequency response of a TIA is very sensitive to variations in load. It is difficult to provide a TIA having a frequency response profile higher than first order. Due to the load sensitivity of the TIA and its low order filtering characteristic, often a higher order filtering stage is required following the TIA, and this higher order filtering stage has a high design complexity and must be designed in close conjunction with the TIA. With high linearity requirements it is very difficult to design a wideband active filter, especially in sub-micron technologies with their low supply voltages.
Alternatively, a current amplifier may be employed to drive a, possibly passive, filter, or load, as it's stability and frequency response are less dependent on load variations than for a TIA. The current amplifier has the advantage of being less sensitive to the load, relaxing the design of subsequent stages of a receiver. Typically, a current amplifier consists of an amplifying input stage driving an output stage, and possibly with an embedded feedback network. The amplifying stage must provide enough gain and bandwidth to drive the load presented by the output stage across the desired bandwidth. Thus, the current amplifier typically requires an input amplifying stage with the same requirements of high gain and large bandwidth as the OPAMP of a TIA. A large bandwidth may be provided by employing a single transistor for the input amplifying stage, but such a single transistor may not provide sufficient gain. Alternatively, the input amplifying stage may employ a classical cascode arrangement of transistors for providing a high gain, but such an arrangement may not provide sufficient bandwidth, due to its high output impedance generating a pole at low frequency.
There is a requirement for an improved high bandwidth amplifier.