Hi,
Take a look at the attached schematic. Most (all?) books will start by telling you to connect V1 (input) in parallel to R2 through a capacitor.
However, this is problematic, because you end up with crippled input impedance unless you bootstrap. Even bootstrapping isn't perfect, since it introduces positive feedback with its own problems. You also absolutely need an input capacitor.
Let's take a look at my schematic again. My arrangement is in fact equivalent to creating a split supply from a single supply, except for the poor ground impedance (high), located midway between R1 and R2. So in fact we treat the circuit as having 3 power rails: +Vcc/2, 0V, -Vcc/2.
The advantages are:
In fact, there's another advantage to this. The ground reference can be made as stiff as we want without reducing input impedance and without consuming large amounts of power, perhaps using an op-amp.
A lot of literature out there insists on single-supply-ish biasing, even though creating a split supply is easy. My question is: why? Am I missing something?
EDIT: Yes, I know I forgot the 0V connection, sorry . Consider R2 lower end connected to Vcc's supply ground.
Take a look at the attached schematic. Most (all?) books will start by telling you to connect V1 (input) in parallel to R2 through a capacitor.
However, this is problematic, because you end up with crippled input impedance unless you bootstrap. Even bootstrapping isn't perfect, since it introduces positive feedback with its own problems. You also absolutely need an input capacitor.
Let's take a look at my schematic again. My arrangement is in fact equivalent to creating a split supply from a single supply, except for the poor ground impedance (high), located midway between R1 and R2. So in fact we treat the circuit as having 3 power rails: +Vcc/2, 0V, -Vcc/2.
The advantages are:
- No input coupling capacitor is needed if we know there's no DC on the line.
- Input impedance isn't sacrificed. If Q1 was a MOSFET, we would benefit from the full gate impedance.
- Coupling stages is easier, since we can make the ground reference stiffer and use it throughout the circuit, in an easy fashion.
- Bootstrapping isn't required, unless very very high impedances are needed.
In fact, there's another advantage to this. The ground reference can be made as stiff as we want without reducing input impedance and without consuming large amounts of power, perhaps using an op-amp.
A lot of literature out there insists on single-supply-ish biasing, even though creating a split supply is easy. My question is: why? Am I missing something?
EDIT: Yes, I know I forgot the 0V connection, sorry . Consider R2 lower end connected to Vcc's supply ground.
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