High Gain, Two-Stage, Fully Differential Audio Amplifier with 0.18μm CMOS Technology

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By assigning suitable voltages to VOD1 and |VOD5|, telescopic amplifier transistor’s dimensions can be calculated. Usually in Eq. (11) K'n is not equal to K'p (). The designers should notice this issue, when they consider the values for overdrive (VOD) voltages (W: MOSFET channel width, L: MOSFET channel length).

(11)

Because of using the same characteristics for M1-M4 and M5-M8:

(12)

(13)

(14)

(15)

For calculating MOSFET’s dimensions such as:

1-AD: Drain diffusion area

2-AS: Source diffusion area

3-PD: Perimeter of the drain junction

4-PS: perimeter of the source junction

Eq. (16) and (17) used.

(16)

(17)

Ls (in Eq. (16) and (17) ) depends on technology, for example for 0.18μm COMS technology Ls is 0.48μm or for 0.13μm CMOS technology it is 0.55μm. M9-M11 used to bias circuit. Note that all of the transistors should be biased in the saturation region. To achieve this issue the designer should consider saturation’s conditions. Solving Eq. (11) requires K' initial value. For finding this value, two methods suggested:

First- Simulate a simple circuit includes a transistor with the same amplifier transistor’s characteristics and using simulation results for finding K'.

Second- Utilize company’s test results about CMOS technology (e.g. MOSIS). Table 1 displays MOSIS simulation results for K'.

Table 1. MOSIS Parametric Test Results

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After finding MOSFET’s aspects ratio (W/L), transistor’s transconductance (gm) can be calculated from Eq. (18).

(18)

Consequently, One-sided gain is obtained from Eq. (22)

(ro is drain-source resistance in MOSFET’s small-signal equivalent circuit).

(19)

(20)

(21)

(22)

3. Circuit Specification’s Goals

The circuit’s specifications goals (that we should be achieved) are:

1- 1.8V supply voltage.

2- Power consumption less than 10mW.

3- Gain higher than 90dB.

4- Differential output swing higher than 2.4V (1.2V one-sided output swing).

5- Sinusoidal input voltage with 1μv peak to peak amplitude.

4. Simulation Results

Table 2 displays the fully differential audio amplifier transistor’s dimensions. Eq. (23) and (24) described The currents consumption of the amplifier circuit.

(23)

(24)

Table 2. Transistor’s Dimensions

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Table 3 shows MOSFET’s parameters simulation results.

Process and model Specifications:

TSMC RF SPICE MODEL

This model has been modified for RF purpose.

Process: 0.18μm Mixed-Signal

SALIIDE (1P6M+, 1.8V/3.3V)

Model: BSIM3 (V3.24)

DOC. NO: T-018-MM-SP-001

VERSION: 1.3.l

Table 3. MOSFET’s parameter’s Simulation Results

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Figure 2 represents one-sided output signals simulation result (out_1 and out_2) which is compatible with Eq. (22). Step response is the time behavior of the outputs of a general system when its inputs change from zero to one in a very short time. The concept can be extended to the abstract mathematical notion of a dynamical system using an evolution parameter. From a practical standpoint, knowing how the system responds to a sudden input is important because large and possibly fast deviations from the long term steady state may have extreme effects on the component itself and on other portions of the overall system dependent on this component. In addition, the overall system cannot act until the component's output settles down to some vicinity of its final state, delaying the overall system response. Formally, knowing the step response of a dynamical system gives information on the stability of such a system, and on its ability to reach one stationary state when starting from another. The step response of the fully differential audio amplifier is displayed in Figure 3. Figure 4 describes the structure used for simulating the step response of the audio amplifier. Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input. In simplest terms, if a sine wave is injected into a system at a given frequency, a linear system will respond at that same frequency with a certain magnitude and a certain phase angle relative to the input. Two applications of frequency response analysis are related but have different objectives. For an audio system, the objective may be to reproduce the input signal with no distortion. That would require a uniform (flat) magnitude of response up to the bandwidth limitation of the system, with the signal delayed by precisely the same amount of time at all frequencies. That amount of time could be seconds, or weeks or months in the case of recorded media. The frequency response and gain-bandwidth value of the fully differential audio amplifier are shown in Figure 5. AC simulation results include:

One-sided output swing: [1.24 V]

Two-sided output swing: [(2×1.24=2.48)]

Power consumption: [(I1+I2+I3+I4+I5+I6+I7)×VDD= (1m+ 0.5m+0.5m+0.5m+0.5m+1m+1m)×1.8=9mW]

Tow-sided output gain:

(25)

MOSFET’s parameters simulation results are presented in Table 3. Table 4 describes simulation results briefly.

Table 4. Output Swing, Gain and Power Results

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Figure 2. One-sided output signals (out_1,out_2)

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Figure 3. Fully differential audio Amplifier step response

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Figure 4. Amplifier step response structure

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Figure 5. Amplifier Frequency response (AC simulation form 100Hz-1.2GHz)

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