Macroeconomics · Unit 4: Financial Sector · 14 min read · Updated 2026-05-11
Central Bank and the Money Supply — AP Macroeconomics
AP Macroeconomics · Unit 4: Financial Sector · 14 min read
1. Core Concepts: Central Bank Role and Monetary Base★★☆☆☆⏱ 3 min
A central bank is the official monetary authority of a sovereign nation, with core responsibilities including controlling the money supply, regulating commercial banks, stabilizing the financial system, and acting as a lender of last resort during banking panics.
MB = C + R
Where $C$ = currency in circulation held by the public, and $R$ = total reserves held by commercial banks at the central bank. Total reserves are split into required reserves ($RR$, mandated by the central bank) and excess reserves ($ER$, reserves held beyond requirement): $R = RR + ER$.
2. The Money Multiplier★★★☆☆⏱ 4 min
In a fractional reserve banking system, banks lend out excess reserves, which creates new demand deposits and expands the total money supply. The simple money multiplier calculates the maximum possible change in the money supply from a change in the monetary base, assuming no excess reserves and no public currency holding.
m = \frac{1}{rr}
The maximum change in total money supply is then $\Delta MS = m \times \Delta MB = \frac{1}{rr} \times \Delta MB$.
If excess reserves or currency drain are given, use the adjusted money multiplier formula, where $er$ = excess reserve ratio and $c$ = currency drain ratio: $m = \frac{1 + c}{rr + er + c}$.
Exam tip: If the AP exam does not mention excess reserves or currency drain, always use the simple $\frac{1}{rr}$ money multiplier.
3. Open Market Operations★★★☆☆⏱ 3 min
Open market operations (OMO) are the most frequently used monetary policy tool, involving the purchase or sale of government securities to change the monetary base and total money supply.
When the central bank buys bonds, it adds new reserves to the banking system, increasing the monetary base and expanding the money supply (expansionary policy). When it sells bonds, it removes reserves, decreasing the monetary base and contracting the money supply (contractionary policy).
4. Other Core Monetary Policy Tools★★★★☆⏱ 4 min
Central banks use two additional core tools to adjust the money supply: the discount rate and interest on reserves.
Exam tip: When the problem gives an excess reserve ratio, always add it to $rr$ in the denominator of the money multiplier.
Common Pitfalls
Why: Students mix up who is transacting; when the central bank sells, banks pay the central bank, removing reserves from circulation
Why: The simple multiplier assumes no excess reserves, which only applies when this is stated or implied
Why: Students confuse the discount rate with market interest rates and assume higher rates encourage lending
Why: Students invert the multiplier because they confuse the required reserve ratio with the multiplier itself
Why: Students mix up how different tools affect the monetary base vs. the money multiplier
Why: Students assume higher interest rates mean more lending, but IOR is paid to banks for holding reserves, not for lending