gồm các mạch khuếch đại D chung, G chung, mạch cascade, mạch S chung với điện trở, với kết nối diode, với tải tích cực hay tải tuyến tính, với mạch thoái hóa cực S, có mô tả chi tiết và các lưu ý khi nghiên cứu
Chapter 2: Amplifier Uniquify Common – Source Amplifier Uniquify SINGLE-STAGE AMPLIFIER • CS Stage CS Stage with Resistive Load – Bias Condition • -> M1 off -> • , -> M1 on • M1 in saturation region (from A->B) -> • with , Because: Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load – Bias Condition CS Stage • When circuit operate in triode region, we have So With Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load – Small-signal Analysis • As shown in the figure is the small signal model for the saturation region • We have while is a signal voltage • And • So we obtain the gain of CS circuit is: Small signal model for the saturation region • So in general: Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load - Small-signal Analysis • So how we maximize the voltage gain of a CS-stage? • We have: • So there are ways to increase the gain: • Increasing and It makes decreases and until -> transistor enter triode region (undesirable) • In case of increasing to take transistor out of triode region -> it’s hard to • Increasing and of course • In general, we cannot control • If we increase -> problem • If we increase by increase or decrease -> but if it’s sufficiently large -> it’s no longer valid In this case, have a bound Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load - Small-signal Analysis For large values of , the effect pf channel-length modulation in M1 becomes significant, the small-signal equivalent circuit is modified as • This is assuming that the transistor is in saturation and • So we have: Where is channel length modulation coefficient Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load – I/O Impedance As we can see by eyes: To maximize the voltage gain, we hope that is infinite If we replace with a ideal current source -> the resistance here will be infinite and the bias condition is satisfied So the gain now is: Value: (5->10) -> sort of a fundamental limit to how much gain we can get from one transistor Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Resistive Load - Summary So for CS stage with Resistive Load, we have If (neglected channel – length modulation) • Gain: If • Gain: I/O impedance: Overall, this stage can amplify the signal but the gain is limited by their parameters that we cannot change too much So it can be used in some small amplifier (we need a small value of gain) Uniquify SINGLE-STAGE AMPLIFIER • CS Stage with Diode-Connected Load - MOSFET resistance • It is difficult to fabricate tightly controlled or reasonable size resistors on chip So, it is desirable to replace the load resistor with a MOS device Diode-connected NMOS devices • The transistor is always in saturation because the drain and the gate have the same potential () • We write: and • So the impedance of the diode-connected is equal to Small – signal equivalent circuit 10 Uniquify SINGLE-STAGE AMPLIFIER • CG Stage – I/O impedance We have: and So almost the stages we have studied before, the input impedance is equal to infinite, but for CG stage, the input impedance is finite We have: As we can see, the output impedance of CG stage is actually identical with CS stage when and the body effect is neglected 67 Uniquify SINGLE-STAGE AMPLIFIER • CG Stage – Special Case Current flowing through is equal to Current through is equal to So that From (1) and (2) we can write: So that: 68 Uniquify SINGLE-STAGE AMPLIFIER • CG Stage – Special case • Input Resistance seen at the source We have: (based on small-signal circuit) Node S: Current flowing through is So 69 Uniquify SINGLE-STAGE AMPLIFIER • CG Stage – Special case • Output Resistance The result is similar to the output resistance when we calculate in CS stage So Which is is the output impedance of the degeneration stage 70 Uniquify SINGLE-STAGE AMPLIFIER • Summary • CS Amplifier: can get a large value of voltage gain by increasing , nonlinear • Source follower: input impedance is large, low impedance load, high voltage gain • CS Amplifier: small input resistance, large output resistance, voltage gain comparable to that of a CS Amplifier 71 Uniquify Cascode Amplifier Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage • Both resistor must be in saturation region • M1 generates small-signal drain current proportional to Vin • M2 routes the current to the load • M1 is the input device (CS Amp with small load resistance) and M2 is the cascode device • Properties: • Same input resistance as a CS Amp • Output resistance much larger than that of CS or CG Amp 73 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage – Cascode Curent Source • As we have studied, we can use MOS devices playing as a current source when they are operate in saturation • So we are trying to arrange those mos devices to be a topology that we can earn the voltage gain as much as possible • We have studied about using a NMOS or PMOS to achieve a current which is nearly constant We also have already mentioned about degeneration mos device so that we can receive the higher output impedance-> the current more constant • In this session, we will try to study about the Cascode Structure – Cascode Current Source 74 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage – Cascode Curent Source • We have a cascode structure and note that both M1 and M2 is identical (same bias and signal current) • We call M1 is cascode/output device; M2 is degenerating/ input device • And source of cascode device must be connected with the drain of degenerating device • If both M1 and M2 are operating in saturation region, M2 can be approximately as a resistor like CS stage with degeneration • So the output impedance is: • If so we get a new choice which has higher output impedance -> good new!! • If is sufficient large -> 75 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage – Transconductance The transconductance is calculated by the changing of over the changing of In small-signal model we can calculate: After finding the output resistance of the circuit, we obtain that: Where is calculated by grounding the independent source and put the voltage source at output -> 76 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage-Bias Condition • M1 in saturation -> That is: • If M1 and M2 are both in saturation, M2 operates as a SF and is determined by • M2 in saturation -> That is • If places M1 at edge of Triode mode, this is the minimum 77 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage - Small-signal analysis • Assuming that both transistors operate in saturation • The drain current produced by the input device must flow through the cascode device -> the current flow from output must be • As we included channel-length modulation • So the gain will be: • Cascode structure -> high output impedance 78 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage - Small-signal analysis • Let’s try some types of cascode structure which can give us a higher output impedance or voltage gain • We use a PMOS device playing as a current source-> see M3 as a resistor • The drain current produced by the input device must flow through the cascode device -> the current flow from output must be • As we included channel-length modulation • So the gain will be: • If is higher than -> this structure has a higher output impedance !! 79 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage - Small-signal analysis • We use a cascode current source to drive the bias current • The drain current produced by the input device must flow through the cascode device -> the current flow from output must be • As we included channel-length modulation • So the gain can be approximated: => A cascode structure need not operate as an amplifier Another popular application of this topology is in building constant current sources The high output impedance yields a current source closer to the ideal, but at the cost of voltage headroom 80 Uniquify SINGLE-STAGE AMPLIFIER • Cascode Stage – Summary • For cascode structure, we earn higher output impedance, higher voltage gain -> better than the former stages • A cascode structure need not operate as an amplifier Another popular application of this topology is in building constant current sources The high output impedance yields a current source closer to the ideal, but at the cost of voltage headroom • When we use cascode structure as a current source, it gives us a more constant current characteristic But in the contrast, we have to pay more power, more voltage for those transistor to operate in saturation • If we use a cascade current source to be a current source in a cascade stage, there is one thing that we must consider is that the voltage will drop when the current flow through each series transistor -> not have enough voltage to maintain all the transistor • When we get the higher impedance ->the potential doesn’t missing while flowing through that load! Because when the output impedance is sufficient large -> the current goes from this stage to another stage is small while the mos device just need the voltage to bias 81 Uniquify