EEET2097 Electronic CircuitsEEET2097 Electronic CircuitsWorkshop 2 (Weeks 6-7)CURRENT SOURCES AND CURRENT MIRRORSSpecial Note:due to the impact of COVID-19, in this workshop you will do theoretical analysis. Youwill also watch a video of circuit implementation and measurement and be provided with themeasurement results. Based on the measurement video and provided data, you will do analysis anddiscussions.1.AIMS(i)Investigate the operation of BJT current sources(ii)Understand the output resistance of BJT current sources(iii)Investigate the use of BJT current mirrors as the active load2.INTRODUCTIONCurrent sources and current mirrors are very important parts of analogue IC designs. The activecurrent sources and mirrors provide biasing currents for amplifier stages and save valuable real estateon the chip compared with discrete element designs. The current mirror can also be used as the activeload in electronic circuits.A BJT based current mirror circuit is shown in Figure 1.Figure 1BJT current mirror.The output current𝐼𝑜is related to the reference current𝐼𝑅??by:𝐼𝑜=𝐼𝑅??1+2𝛽(1)where𝛽is the current amplification factor of the BJT. Assume𝑉𝐴2is the Early voltage of transistor Q2,the output resistance (taken from Q2) can be described as:𝑅𝑜??≈𝑉𝐴2𝐼𝑜=𝑉𝐴2𝐼𝑅??(2)
EEET2097 Electronic Circuits3.WORKSHOP PROCEDUREIn this workshop, we will use the pre-designed differential amplifier card. Current mirror and currentsource are part of the card. The circuit schematic is shown in Figure 2.Figure 2Laboratory differential amplifier circuit schematic.The differential amplifier needs to be installed to a card frame to use (please refer to the measurementvideo). The numbers with a circle around them in Figure 2 correspond to the numbered sockets at thebase of the card frame. We will refer to these numbers as “socket X”.3.1DC BiasingIn this part, we will focus on the DC biasing of course source. Note the inclusion of:•ThelinkA1;•The1 MΩ resistorE2.As shown in the measurement video (video Part 1-1), we connect socket 10 in Fig. 2 to ground (therebysettingVB1= 0V) and connect a DC current meter between sockets 6 and 7. We also connect DC supplyvoltagesV+(+10 Volts) andV–(–10 Volts) as marked (i.e., sockets 16 and 1, respectively). Note thatwe use the dual DC supply on the workbench to generate ±10V supply voltages.We then adjust the500 KΩ potentiometerin the circuit to set the emitter currentIEof thetransistorQ1 toIE= 1 mA.