Figure 1 (a) shows the basic idea of a conventional current generator circuit, which its performance can be improved by adding a resistance at the source terminal of transistor M4 [8]. This circuit forms a positive feedback loop and will be stable for a loop gain less than unity. This structure is very useful to design a transconductance cell with a self-biasing topology.

Figure 1 (b) shows the proposed self-biasing transconductance cell [9]. As is seen, transistor M0 is added as the tail current to create a virtual ground at the source of M1-M2, which results in a special differential pair. Based on the analysis done in [9], having a tail current will increase the common-mode-rejection ratio (CMRR) of the structure. This circuit can only be converted to a bulk-driven transconductance cell because there is no more idle terminal to be used as the input node except bulks. Therefore, the bulk terminals of P-type transistors M1-M2 are used as the input nodes that offer a high input dynamic range.

To stabilize the proposed structure, an adjustable current source, M5, should be paralleled with one of the non-diode-connected transistors M2-M3. Actually, without employing M5, the proposed structure provides an infinite output impedance casing an unstable behavior in the transient response. Eq. (1) expresses the impedance seen from both outputs where its amount is adjusted by the transconductance of M5, *gm*5, and the pole associated with output nodes can be moved by tuning *V*b2.

$${R_{out \pm }} \approx \frac{1}{{g{m_1}\left( {1 - \frac{{g{m_2}\left( {g{m_1} - g{m_5}} \right)}}{{g{m_1}g{m_4}}}} \right)}}$$

1

where *gm**i* represents the transconductance of the corresponding transistors, and (*gm*1-*gm*5) = *gm*3. Therefore, to achieve a finite positive impedance at the output, *gm*5 should have a non-zero value. It is obvious that the proposed structure benefits from a high output impedance, which results in a higher transconductance and a larger DC gain in comparison with a conventional bulk-driven structure. In addition, the proposed cell provides a double-ended structure, while the output common mode voltages are controlled by its self-biasing topology without employing any common-mode feedback (CMFB) circuit. By employing two of the proposed cells, a self-biasing high-performance transconductance amplifier can easily be designed as a tunable high dynamic range circuit.