2. MOSFET Source Follower (Common Drain Amplifier)

3. MOSFET Amplifier with Active Load

4. Cascode Amplifier using MOSFET

5. What is Current Mirror ? MOSFET Current Mirror Explained

The post MOSFET Tutorials appeared first on ALL ABOUT ELECTRONICS.

The current mirror is an analog circuit that senses the reference current and generates the copy or number of copies of the reference current, with the same characteristics. The replicated current is as stable as the reference current source. The replicated current could be the same as the reference current (I_copy = I_REF), or it could be either multiple or fraction of the reference current. (I_copy = N*Iref or I_copy = (1/N)*I_REF).

Fig.1 Basic function of Current Mirror Circuit

Why Current Mirrors are used?

Current Mirrors are particularly useful in the integrated circuits, for biasing the amplifiers. The advantage of biasing the amplifiers with the current source is that it provides a high voltage gain and good biasing stability. This current source can be generated using a simple PMOS transistor or using a MOS transistor in cascode configuration (to achieve higher gain) as shown in Fig.2.

But this type of MOS current source (shown in Fig.2) is susceptible to the change in the biasing voltage and the change in temperature. Moreover, the integrated circuit may contain hundred or thousand of such amplifiers. To bias all the amplifiers with precise biasing voltage is another challenge. So, to overcome all these problems, in integrated circuits, one stable current source is fabricated within IC, and using the current mirror the multiple copies of the stable current source is generated (which can be used to bias the amplifiers)

MOSFET- Current Mirror

Fig.3 shows a current mirror circuit using the NMOS transistor. The reference current is converted to the voltage using diode connected transistor and the same is applied between the gate and the source of the another MOSFET.

Fig.3 Current Mirror Circuit using NMOS transistors

The relation between the I_D1 and I_REF can be given by the following expression.

By changing the W/L ratio of the two transistors, the current which is fraction or multiple of the reference current can be generated. The only thing which needs to be ensured is that, the MOSFET should operate in the saturation region.

Effect of Channel Length Modulation on Current Mirror

So far during the discussion, the effect of channel length modulation was neglected. If the channel length modulation effect is also considered, then as shown in Fig. 4, as the drain to source voltage (V_DS) of the MOSFET increases, the drain current also slightly increases.

Fig.4 The effect of channel length modulation on drain current of the MOSFET

Considering the channel length modulation effect, if V_DS of MOSFET M₁ changes by ΔV_DS then the drain current I_D1 also changes to I_D1 + ΔI_D1. The same is shown in Fig. 5.

Fig. 5 Effect of Channel Length Modulation on Current Mirror

The effect of channel length modulation can be reduced by increasing the length of the channel for the given W/L ratio.

PMOS Current Mirror

Fig. 6 shows the implementation of current mirror using the PMOS transistors. In PMOS current mirror, the source terminals for both transistors are connected to Supply voltage Vdd.

Fig. 6 Current Mirror using PMOS transistors

The relation between the I_D1 and I_REF can be given by the same expression.

The only thing which needs to be ensured is that M1 should operate in the saturation region. Or in other words, V_SD1 ≥ V_SG – |V_TP |, Where V_TP is the threshold voltage of the PMOS transistor.

Current Steering Circuit

Fig. 7 shows the current steering circuit, which can be used to drive different amplifiers. And using this circuit is possible to generate a bias current which is multiple or the fraction of the reference current source.

Fig. 7 Current Steering Circuit Example

For example, let’s say (W/L)₁ = 2 x (W/L)_{REF ,} (W/L)₂ = (W/L)₃ = 3 x (W/L)_REF and (W/L)₄ = 6 x (W/L)_REF.

Therefore, I_D1 = 2 x I_REF and I_D2 = 3 x I_REF. The same current I_D2 will also flow through the transistor M3. Therefore, I_D3 = I_D2. And since (W/L)₃ = 3 x (W/L)_REF and (W/L)₄ = 6 x (W/L)_REF, I_D4 = 2 x I_D3 or I_D4 = 6 x I_REF. (assuming μ_n C_ox = μ_p C_ox, and overdrive voltage for M2 and M3 are equal)

So, in this way, by changing the W/L ratios, it is possible to generate a multiple or fraction of the reference current. And the same can be used to drive different circuits. For example, current I_D1 can be used to drive a source follower circuit and current I_D4 can be used to drive a common source amplifier. The same is shown in Fig. 8.

Fig. 8 Curent Mirror as an active load for the amplifiers

Typically in the integrated circuits, the amplifiers are biased as shown in Fig. 8. Where current ID4 and ID3 are derived using the current mirror circuits. This type of arrangement provides good biasing stability. But to further improve the gain of the amplifier, the cascode current mirror circuits are used and the same is used as an active load with the MOS amplifiers.

For more information, please check this video on Current Mirror:

The post What is Current Mirror? MOSFET- Current Mirror Explained appeared first on ALL ABOUT ELECTRONICS.

The Cascode Amplifier is the combination of the common source (Common Emitter for BJT) and the Common Gate Stage (Common Base for BJT). As shown in Fig. 1, the input is applied to the common source amplifier.

Fig.1 Cascode Amplifier

The transistor M1 is also known as amplifying transistor. And the output of this transistor is fed to the common gate stage (M2). The output of the cascode amplifier is measured at the drain terminal of the common gate stage (M2). For a time being here, the load is not shown. But the load could be a passive resistive load or it could be an active load like a resistor.

The Cascode amplifier provides high intrinsic gain, high output impedance and large bandwidth.

Output Resistance of Cascode Amplifier:

The common gate stage multiplies the output resistance of the common source stage. If ro1 is the output resistance of the transistor M1, then the output resistance seen from the drain terminal of the M2 is approximately g_m2r_o2r_o1.

Fig. 2 The output resistance of the Cascode Amplifier

Because of its higher output impedance, the intrinsic gain of the cascode amplifier is also very high.

Intrinsic Gain of Cascode Amplifier:

The intrinsic gain of the cascode amplifier can be found by finding the overall transconductance of the cascode stage. If Gm is the transconductance of the cascode stage and Ro is the output resistance of the cascode amplifier then intrinsic gain Ao = – Gm Ro

Fig.3 The output equivalent circuit of the Cascode Amplifier without load

For the cascode stage, the transconductance Gm ≈ g_m1 and Ro ≈ g_m2 r_o2 r_o1. Therefore, the intrinsic gain |Ao| = g_m1g_m2r_o1r_o2. The intrinsic gain of the Cascode amplifier is significantly higher than the common source amplifier. The overall voltage gain of the cascode configuration depends on the load.

Cascode Amplifier with Resistive Load

Fig. 4 Cascode Amplifier with resistive load

Fig.4 shows the cascode amplifier with the resistive load and the output equivalent circuit. In this case, the voltage gain |Av| ≈ g_m1 ( R_o|| R_D). Where Ro is the output resistance of the cascode stage. Typically Ro >> R_D. Usually, R_D is in kΩ, while Ro is in MΩ. Therefore, |Av| ≈ g_m1 R_D. It is typically the same as the common source amplifier with resistive load.

Cascode Amplifier with current source as a load

The higher gain can be achieved using the active load. As shown in Fig.5, if the active load is the ideal current source then, the voltage gain |Av| = g_m1 Ro.

Fig.5 The cascode amplifier with ideal current source as a load.

But the actual current source has finite output resistance. And due to the finite output resistance, the overall voltage gain reduces. Fig.6 shows the cascode amplifier with PMOS as a current source. Where Ro1 is the output resistance of the cascode stage and ro2 is the output resistance of the PMOS.

Fig. 6 The Cascode Amplifier with PMOS as a current source

With PMOS current source as a load, the voltage gain |Av| ≈ g_m1 r_o2. Because the output resistance Ro1 of the cascode stage is much greater than the output resistance of the PMOS. Typically, with this arrangement, the achieved voltage gain is similar to the intrinsic gain of a common source amplifier. To further improve the gain, the cascode current source can be used as a load with cascode amplifier.

Cascode Amplifier with Cascode current source

Fig. 7 shows the typical Cascode current source using PMOS transistors.

Fig. 7 Cascode current source using PMOS transistor

The output resistance seen from the drain of M3 transistor is approximately equal to g_m3 r_o3 r_o4. Which is typically much higher than the output resistance of the single PMOS transistor.

Fig. 8 shows the cascode amplifier with the cascode current source as a load.

Fig. 8 Cascode Amplifier with Cascode Current Source

With Cascode current source as a load, the voltage gain of the cascode amplifier |Av| ≈ g_m1 ( R_on || R_op)

Where, R_on = g_m2 r_o2 r_o1 is the output impedance of the cascode amplifier stage, and Rop = g_m3 r_o3 r_o4 is the output resistance of the cascode current source. Using the cascode amplifier along with the cascode current source, the gain of the amplifier can be improved significantly. And typically, this configuration is used in the integrated circuits to achieve a large gain.

For more information, and for small-signal analysis, check this video:

The post Cascode Amplifier using MOSFET Explained appeared first on ALL ABOUT ELECTRONICS.

In MOSFET amplifier circuits, instead of a passive resistor, the active component like a MOSFET or MOSFET circuit (Current Mirror) is used to increase the gain of the amplifier. This active component or the active circuit is known as the Active Load.

Fig.1 MOSFET Amplifier with Active Load

Why Active Load is used in the circuit design?

The Active Load is usually used in the integrated circuits where the size and power consumption are major constrain. Also, in the integrated circuits, fabricating a resistor requires a lot of space. And therefore, instead of a resistor, the active load is used. The active load also helps in increasing the voltage gain of the amplifier. To understand this point, let’s take the example of a simple common source amplifier with the passive load.

Fig. 2 Common Source amplifier with Passive Load

As shown in Fig.2, the gain of the common source amplifier is |Av| = gm*(R_D || ro).

Where, ro is the output resistance of MOSFET. If ro >> R_D then |Av| ≈ gm*R_D

To increase the gain, either R_D or g_m needs to be increased. But as R_D increases, the voltage drop across R_D also increases and hence, the available voltage at the drain terminal reduces. At one stage, the MOSFET may come output of the saturation.

Similarly, by increasing the drain current ID, the transconductance (gm) can be increased. But as the drain current increases, the power dissipation in the circuit increases. Also, with the increase in the drain current, the voltage drop across the drain resistor will increase. And at one point, MOSFET may come out of the saturation.

Of course, by increasing the power supply voltage, the MOSFET can be kept in the saturation region and it is possible to increase the gain by some extent. But that is not an option in the modern integrated circuits where the supply voltage range is shrinking day by day. All these problems can be eliminated using the active load.

Ideal Current Source as an Active Load

Fig.3 Biasing the MOSFET with Current Source (Current Source as an active load)

The amplifier can be biased using the constant current source. The current source is the example of active load. If the current source is ideal then there are couple of advantages.

1. Bias current remains stable irrespective of the changes in the external circuit parameters like temperature
2. The ideal current source has infinite output impedance.

In the AC equivalent circuit, the ideal current source can be replaced by the open circuit. And the voltage gain of the amplifier |Av| = gm*ro, where ro is output impedance of the MOSFET.

gm*ro is known as the intrinsic gain of the amplifier. It is a maximum obtainable gain for the given amplifier configuration. Using the current source as a load, there is a significant improvement in the voltage gain. Typically, with the current process technology, it is possible to obtain the gain in between 20 to 50. Typically, in the integrated circuits, the current source for the biasing is generated using the current mirror circuit. And since it is a non-ideal current source, it has some finite output impedance. Because of its finite output impedance, the gain further reduces.

Fig. 4 Actual Current Source (with finite output resistance)

In the small-signal equivalent circuit, the actual current source can be replaced with its output impedance. (As shown in Fig. 5)

Fig.5 Current Source will get replaced by its output impedance in the AC equivalent circuit

And the voltage gain |Av| = g_m*(ro1 || ro2). Hence, because of the finite output resistance of the actual current source, the voltage gain reduces. To increase the voltage gain, the output resistance of the current source needs to be increased. Typically, to achieve a large voltage gain, the cascode amplifier along with the cascode current source is used. In the upcoming articles, the cascode amplifier and current mirror circuits will be discussed.

The post MOSFET Amplifire with Active Load appeared first on ALL ABOUT ELECTRONICS.

The Common Drain Amplifier has
1) High Input Impedance
2) Low Output Impedance
3) Sub-unity voltage gain

Since the output at the source terminal is following the input signal, it is also known as Source Follower.

Because of its low output impedance, it is used as a buffer for driving the low output impedance load.
Often in multistage amplifiers, while driving low impedance load, the source follower is used as an output stage.

Fig. Common Drain Amplifier (Source Follower) with Biasing Circuit

In the given PDF link, through a small-signal analysis, the expression of input impedance, the output impedance, and the voltage gain of this common drain amplifier is derived.

The post MOSFET- Source Follower (Common Drain Amplifier) appeared first on ALL ABOUT ELECTRONICS.

The post MOSFET- Common Gate Amplifier appeared first on ALL ABOUT ELECTRONICS.