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Analog CMOS Integrated Circuit Design Current Mirrors Jawdat Abu-Taha Department of Electrical and Computer Engineering Islamic University of Gaza [email protected] 1 Applications of Current Sources 2 Simple Resistive Biasing for Current Source 3 Problems • Output current depends on: – Supply – Process – Temperature • What if the bias voltage is independent of supply voltage? • Is there a way of generating reliable currents? 4 Basic Idea Typically we assume that one precisely defined current source is available and other current sources copy their current from this precise source. Iout is a function of gate-source voltage 5 Basic Idea This structure is called current mirror 6 Question • What happens if the two transistors in the basic current mirror have different sizes? 7 Example Assuming all the transistors are in saturation region, find Iout: 8 Current Mirrors: Amplifier Bias Example 9 Current Mirrors: Signal Amplification Example • Find the small signal voltage gain of the following circuit. 11 Effect of Channel Length Modulation 12 Cascode Current Mirror 13 Cascode Current Mirror 15 Cascode Current Mirror SM EECE488 Set 5 - Current Mirrors 16 Cascode Current Mirror 17 Cascode Current Mirror Biasing 19 Cascode Current Mirror Biasing 20 Current Mirror Biasing 21 Basic Circuit to Generate Supply Independent Current 22 Supply Independent Current 23 Supply Independent Current 25 Start-up Problem 26 Start-up Problem 27 Active Current Mirrors 29 Active Current Mirrors in Differential to Single-Ended Amplifiers 30 Differential to Single-Ended Amplifiers 31 Calculation of Gm 32 Small-Signal Gain 33 Common Mode Characteristics 34 Common Mode 35 Common Mode 36