EEET2097 Electronic Circuits Workshop 2 - 2020.pdf -...

EEET2097 Electronic Circuits
EEET2097 Electronic Circuits
Workshop 2 (Weeks 6-7)
CURRENT SOURCES AND CURRENT MIRRORS
Special Note: due to the impact of COVID-19, in this workshop you will do theoretical analysis. You
will also watch a video of circuit implementation and measurement and be provided with the
measurement results. Based on the measurement video and provided data, you will do analysis and
discussions.
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 load
2. INTRODUCTION
Current sources and current mirrors are very important parts of analogue IC designs. The active
current sources and mirrors provide biasing currents for amplifier stages and save valuable real estate
on the chip compared with discrete element designs. The current mirror can also be used as the active
load in electronic circuits.
A BJT based current mirror circuit is shown in Figure 1.
Figure 1 BJT current mirror.
The output current 𝐼
𝑜
is related to the reference current 𝐼
𝑅??
by:
𝐼
𝑜
=
𝐼
𝑅??
1+
2
𝛽
(1)
where 𝛽 is the current amplification factor of the BJT. Assume 𝑉
𝐴2
is the Early voltage of transistor Q2,
the output resistance (taken from Q2) can be described as:
𝑅
𝑜??
≈
𝑉
𝐴2
𝐼
𝑜
=
𝑉
𝐴2
𝐼
𝑅??
(2)

EEET2097 Electronic Circuits
3. WORKSHOP PROCEDURE
In this workshop, we will use the pre-designed differential amplifier card. Current mirror and current
source are part of the card. The circuit schematic is shown in Figure 2.
Figure 2 Laboratory differential amplifier circuit schematic.
The differential amplifier needs to be installed to a card frame to use (please refer to the measurement
video). The numbers with a circle around them in Figure 2 correspond to the numbered sockets at the
base of the card frame. We will refer to these numb ers as “socket X”.
3.1 DC Biasing
In this part, we will focus on the DC biasing of course source. Note the inclusion of:
• The link A1;
• The 1 MΩ resistor E2 .
As shown in the measurement video (video Part 1-1), we connect socket 10 in Fig. 2 to ground (thereby
setting V
B1
= 0V) and connect a DC current meter between sockets 6 and 7. We also connect DC supply
voltages V+ (+10 Volts) and V – ( – 10 Volts) as marked (i.e., sockets 16 and 1, respectively). Note that
we use the dual DC supply on the workbench to generate ±10V supply voltages.
We then adjust the 500 KΩ potentiometer in the circuit to set the emitter current I
E
of the transistor
Q1 to I
E
= 1 mA.