To demostrate the uselfuness of the ACM model, we presented a handy-design example.

DESIGN EXAMPLE

In this example, we designed a folded cascode operational amplifier using the ACM equations for comparison with simulation results from PSP. The specification and simulation results for the op amp are given in Table I

Table I- Specifications and simulations results of the folded-cascode amplifier.

whereas the topology is shown in Fig. 1.

Fig.1 Folded-cascode op amp.

The op amp was designed using the ACM parameters extracted from MOSFET characteristics simulated with version 6.6, release 2005.3, of ELDO using the level 70 (PSP) model with default parameters. In the default technology, the p-channel and n-channel transistors are “symmetric” devices.

The first parameter determined in our design was the transconductance gm1 of the input transistors from

                                 (1)

which gives g_m1= 314µA/V (GBW=10MHz and C_L=5pF). The minimum current that meets the transconductance specification is obtained in weak inversion, i.e., making in the expression of g_m/I_D in Table II.

Table II- Design-oriented expressions for the long-channel MOSFET in saturation.

One can easily verify that, for ,. Therefore, the minimum current of the input transistors M1 and M2 is I_D1min= 9.1µA (we have used n=1.12 and thermal voltage =25.9 mV). In our design I_D11= I_D12=I_D15. In this way, both the rising and falling slew rates are equal to I_D5/C_L. Therefore, to comply with the required slew rate spec, we need I_D5=2I_D1>20 µA. We chose the bias current I_D5=27 µA. Using I_D1=13.5 µA together with g_m1= 314µA/V we find the inversion level if. The inversion levels of all transistors, except one in the bias network, equal to 3. The advantage of having such a low inversion level is that the saturation voltage of the transistors is around 130 mV, a relatively small value. Once we had chosen the drain current of the transistors, we could readily find the aspect ratios using the expression in Table II, which gives L/W as a function of the drain current and transconductance, besides the technological parameters. In the Table III is shown the transistor dimensions of the Folded-cascode op amp and bias network.

Table III- Transistor Dimensions.

The bias network [4] is driven by a current source I_BIAS=13.5 µA . The aspect ratios of MP1 (MP2, MP3) and MN3 are equal to one half the aspect ratios of M5 and M11, respectively.

Fig.2 Bias network of the folded-cascode op amp

INTRODUCTION