Single-transistor circuits Buffer amplifier

1 single-transistor circuits

1.1 impedance transformation using bipolar voltage follower
1.2 impedance transformation using mosfet voltage follower
1.3 chart of single-transistor amplifiers

single-transistor circuits

figure 4: top: bjt voltage follower bottom: small-signal, low-frequency equivalent circuit using hybrid-pi model



figure 5: top: mosfet voltage follower bottom: small-signal, low-frequency equivalent circuit using hybrid-pi model

other unity gain buffer amplifiers include bipolar junction transistor in common-collector configuration (called emitter follower because emitter voltage follows base voltage, or voltage follower because output voltage follows input voltage); field effect transistor in common-drain configuration (called source follower because source voltage follows gate voltage or, again, voltage follower because output voltage follows input voltage); or similar configurations using vacuum tubes (cathode follower), or other active devices. such amplifiers have gain of less unity, difference small , unimportant.

impedance transformation using bipolar voltage follower

using small-signal circuit in figure 4, impedance seen looking circuit is

r

i
n


=



v

x



i

x




=

r

π


+
(
β
+
1
)
(


r

o




|


|



r

l



)


{\displaystyle r_{in}={\frac {v_{x}}{i_{x}}}=r_{\pi }+(\beta +1)({r_{o}}||{r_{l}})}

(the analysis uses relation gmrπ = (ic /vt) (vt /ib) = β, follows evaluation of these parameters in terms of bias currents.) assuming usual case ro >> rl, impedance looking buffer larger load rl without buffer factor of (β + 1), substantial because β large. impedance increased more added rπ, rπ << (β + 1) rl, addition not make difference

impedance transformation using mosfet voltage follower

using small-signal circuit in figure 5, impedance seen looking circuit no longer rl instead infinite (at low frequencies) because mosfet draws no current.

as frequency increased, parasitic capacitances of transistors come play , transformed input impedance drops frequency.

chart of single-transistor amplifiers

some configurations of single-transistor amplifier can used buffer isolate driver load. digital applications, nmos voltage follower (common drain) preferred configuration. these amplifiers have high input impedance, means digital system not need supply large current.